To want to completely change what my DC eat even when DH thinks I’m making a fuss?

Yanbu. And you are right. You are at an age and tipping point for them where if you make changes for the better it will have a lasting benefit for your DC for the whole of their lives. Stay strong. I would start with the snacks - just don’t have biscuits in the house or if you do get ones they don’t like so they have to actually be hungry to have one (rich tea or a plain digestive). Also snack food is convenience so make it easier to have a healthy choice - simple things like having a snacking plate where the apple or pear or carrot is cut into pieces for the afternoon snack worked. They might go urgh and not eat it but if you leave it out they will come back if they are hungry. For the good dinners - I would always say you have to try it before I give you something else - so fish. A mouthful and then I’ll do toast. Sometimes they change their mind once they have tried it.
good luck. Ignore your husband and know you are doing a great thing for your DC. Mine are adult teens now and thank me for it. They didn’t at the time.

YANBU.
There is a balance to be had here.
It isn’t terrible to give your child chips and squash on occasion, a couple of biscuits as a snack, Coco pops on a Saturday morning. But that should be the exception, not the rule. Most days should be centred around more veggies, whole foods, fibre, lower sugar etc.
Focus on how many colours they’re eating and avoiding too much beige.
But freezer food certainly has its place and quick dinners will become a necessity as they get older and will maybe have clubs in the evenings after school etc. This could be a batch cooked dinner that you can ping in the microwave, rather than nuggets every time.
Not everyone has time to home make everything but you can get in better habits such as making extra dinner and freezing it for another day.

It’s also worth mentioning that if you make biscuits/choc etc forbidden/a rarity you run the risk of making them more enticing and your children not being able to control themselves around “treats”. A biscuit with a yoghurt or some fruit is fine. And water should be served with every meal and snack.

Well done for identifying the issue and making steps to deal with it

Well done to you for recognising and wanting to make a change.
Unfortunately you are going to massively struggle unless you get DH on board. I am not sure how to help with that as I ended up divorcing my ex before managing it (not for that reason but it sort of fed into it as he didn't have a backbone with the kids and let them get away with all sorts).

One small tip - I used to serve mine "starters" which was a small plate of whatever veg they do like - so mine would have e.g. cucumber fingers, carrot fingers, baby corn - given to them when playing before dinner. They were half distracted by TV or games and already hungry, so would usually eat it all. Meaning, they'd already had a good amount of veg before even starting "dinner".

It does sound like they’re having mostly carbs and fats. I’d cut down on the carbs and fats, and add in more protein and fibre. Specific ingredients don’t matter, it all comes down to macros at the end of the day, but the high carb foods are the things like potatoes, bread, biscuits, pasta, rice (beige foods) and the fats are things like dairy, nuts, seeds, and fatty meats. Your proteins are things like fish, lean meat, nuts, and tofu, so they should have more or those. But go easy on the nuts as they are also high in fat.

Fruit and veg are important for vitamins but if you take a multivitamin honestly you can live without it. The other huge benefit of fruit and veg though is that it is low calorie for high volume so you can eat a lot without risking getting fat. You didn’t say weight was a concern for you, just health, but if you are worried about weight then filling them up on fruit and veg is a good way to reduce their overall calorie intake.

I don’t know much about teeth but it sounds like that’s also a concern for you. Definitely take your dentist’s advice and limit sugar, make sure they’re brushing properly etc. They don’t want issues with their teeth as they get older, it’s expensive if nothing else!

Its a problem

You know it which is why you wrote it here.

We do a rough meal plan for the week.

Kids eat boiled potatoes, pasta or noodles plus a protein (chicken, beef, pork, fish) and a few veg (carrots peas sweetcorn green beans or sticks of cucumber on the Side)

We also do shepherds pie and pasta bake a lot.

Dessert is yoghurt a piece of fruit or all butter short bread biscuit (these are 3 ingredients from m&s)

@boltmelon
Well done for making the change early.
My 22yr old is horrified his GF doesn't have a piece of fruit after each meal, and DD similarly that her BF rarely has veg. Growing up with good habits sets them up for life.

Get them involved.
Get them chopping up can cucumber sticks or peeling carrots (DD16 will still ask to help with if I'm doing that).
Bake together.
Meal plan together.
Everyone gets to choose one dinner per week so they get their favourite once, but they have to eat everything.
Took a while for mine to tell the difference between 'i don't like it' and 'its not my favourite but I'll eat it'.
Don't make different things if they don't eat what is provided.

Don't let DH undermine you. You are 100% doing the right thing.

You’ve recognised a problem and want to fix it. There are work arounds. Mine have option of weetabix, Bluey cereal (no added sugar) plain Cheerios or porridge for breakfast. Fruit afterwards. If they want coco pops because of their cousins/other kids eating them I say it doesn’t have enough energy for the day and they can have it in the evening if they want. They rarely do by the time evening comes along. I have the small variety pack one’s in. For dinner sometimes I’ll do kids pasta and organic tomato and veg sauce, cheese grated on top, peas in it. Ten mins to make. For lunches we do a lot of ‘treat plates’ - sandwich, fruit , yoghurt, avocado slices, cheese slices, chicken slices, cherry tomatoes and cucumber plus a biscuit or 3-4 chocolate buttons. They don’t seem to realise only one thing is the treat and usually polish it all off. Mine love raw carrots as a snack.

we do stews and Shepards pie and chicken mash and veg too but I find they usually eat more of the above. Also limit the snacks bar fruit. If hungry before bed I’d go with a warm weetabix rather than toast. Nothing wrong with baked beans fish fingers and a healthier chip too the odd time ( like the strong roots brand).

Your kids are eating huge quantities of processed junk.

And the baby teeth bit is irrelevant - no kids should have tooth decay by the age of 5. That's quite shocking.

And by referring to your 5 year old as being solid - it does sound as if he is over weight. In reception they will take their height and weight and you will get the letter home saying your child is overweight. My brother got this as he had a fear of his DS being hungry so was always letting him have snacks.

I would stop the crisps and biscuits altogether. Snacks can be fruit, nuts, or wholemeal bread/toast or a bowl of healthy cereal. Get rid of the chocolate cereals too.

Breakfast: wholemeal toast, porridge, or lower sugar cereals. My DS likes banana milkshakes made in the blender, or we put in the frozen fruit (buy in bags from Lidl).

Meals can be batch cooked so it halves how much you have to cook. e.g make two cottage pies and freeze one for the following week.

Pasta sauce is easy - tin of quality tomatoes, onion, garlic, a carrot, veg stock and basil then blend til smooth. Freeze in small portions then you just need to cook pasta and defrost a cube of sauce - easy quick meal.

It's brilliant that you're trying to make the changes now.

And stop worrying about kids going hungry. If they really don't eat a meal then they get something boring like some plain rice. Don't give them extra chips if they don't eat the fish and veg!!!

Wow.
I don't think you are over reacting. It's important to make good habits and I think it might be quite hard with a DH who sounds very opposite on view.

For comparison we are a family of four with a three year old and a seven year old. They eat a lot but quite balanced.
Today they had porridge with chia seeds, flax seeds and honey and using whole milk.
Homemade spinach and cheese muffins (from the gruffalo cookbook - which I batch cook and freeze) with cream cheese and olives for the 7 year old. The 3 year old had a plum

Snack was homemade banana and cinnamon muffins, again batch cooked and frozen and I just defrost for the weekend, the make a good swimming snack.
The 3 year rold also had raisins and a bar.
The snacks were after different sports activities.

Lunch was eggy bread, crudité and Quorn sausages and a boiled egg for the three year old. I also had cooked a butternuts squash and chickpea curry that neither of them entertained but both tried which is what I want them to do.

There will no doubt be another fruit snack - apples or bananas and further crudité. Could be the same as lunch or different. And probably a bag of crisps.

We are making homemade pizza for dinner which is frozen ready made dough. And I'll have crudité again. I finds cold veg easiest to get down them. But sugar snap peas and mange tout work really well too

We are not opposed to takeaways but it's a once a month thing. It used to be more and we had similar challenges that you describe.

Id say no chocolate cereal or biscuits in the house. We only have bran flakes and Weetabix and I would prefer porridge but that isn't always realistic in the week. We also never do toast. Of we do it's lunch with eggs etc.

Good luck

Firstly, well done for recognising this needs to change. It absolutely does. Your husband is an utterly terrible parent, who frankly is bordering on neglect of his children. Your suggestions aren’t ’a diet’, it’s just food rather than completely processed crap which has zero nutritional value. It isn’t just about their weight, their behaviour and ability to learn is massively affected by not having a single morsel of nutrition. Also, your dh is a fool about the teeth - it surely is obvious it isn’t just affect their actual teeth, but also their gums and mouth health. If he can’t get a job, and he also can’t parent, I guess it’s a difficult decision to make for the two of you to switch roles and you be the earner.