To resent carrying all of the mental load of catering for DSD's dietary requirements

Get a couple more small pans and cook carbs, meat and veg separately, then everyone takes what they want. Eg
Pasta, meat, tomato or veggie sauce in three pans, grated cheese on the side.
Wraps with meat, salad, and cheese in sep pots.
Rice, soy sauce meat, soy sauce veg.
its no more work putting ingredients into separate pots then the same pot. I had a fussy eater and it worked for me.

So would the fussy kids! Unless we were talking about ARFID but it doesn't seem to be.
What doesn't sit well is the fact so many replies say they wouldn't follow the restrictions for her, but don't mention the fact it's only really a problem because the other children are fussy and won't eat the same thing.

ETA: That's not to say OP isnt right to be fed up, there's a definite husband problem here!

Yes. I wouldn't be able to do it as a non Jew - I know the rules as I have worked in a Jewish institution but they are not easy to adhere in a normal household or supermarked.

Presumably Muslim or Jewish or similar religion where not following food laws is just not possible.
DH should cook for her and him and you can deal with other DC and you.
Try to cook one meal and adaptions where at all possible.
Those seem the only sensible SOLUTIONS
Edited as saw your update, get her lazy git of a dad to cook for her, presumably she can have spaghetti bol without cheese and your kids can have cheese. You can be strict if you don't have separate prep and cooking areas so this must be possible. Same with plant based pastry on pies, mash made with olive oil etc

Keeping Kosher is far more involved than just not eating pork and waiting 6 hours between meat and milk dishes. All your equipment and appliances need to be blessed, the kitchen and all equipment for food and drink needs to be made Kosher via very specific cleaning and blessing. Any new kitchen equipment used for cooking or eating needs to be taken to a mikvah and all foods she eats checked for Kosher compliance.

The efforts you are making are not following the Kosher Laws properly anyway, so if it is too much, I would stop worrying and just do what you can. Your SD's Mother cannot dictate that you follow this practise and since her daughter is not keepings Shabos of her own accord and is not orthodox there is no way she can force you to comply. It is a practise that she should decide about for herself in the future and her father needs to support you by saying that to his ex wife.

food isn’t a religious belief… is it?
If there was a supply chain issue / war / rationing (I appreciate there isn’t) people who subscribe to religions would eat what was available to them.

If she's upset that the other kids get something she doesn't then it's very simple to explain that no one is stopping her. But if she doesn't want to eat it she doesn't get to complain that other people do.

Surely if you choose to be with a partner who is Jewish and has a Jewish child you should be willing to accommodate the dietary requirements?

Some of the comments here are awful, like its pandering to a fad, and suggesting 7 year old should be made to feel like an outcast at mealtimes

That's massively restrictive and really not your job.

She isn't Jewish. Her mum has chosen for her to be Jewish. It doesn't matter if it "passes down" it's a choice the parent makes for the child. Until she decides it for herself when she is able to make those decisions, it is simply that her mother wants her to be Jewish.

If her dad wants to follow her mothers religion that's fine. But that's his responsibility.

Yours and your children's diet should not be dictated by your husband's ex wife.so no, everyone shouldn't be vegetarian or vegan. You tell him from now on you are just cooking for everyone. If she wants something different or he wants her to have something different then that's his responsibility to sort out.

You didn't get a choice in her diet, so you are not responsible for maintaining it.

I would ask her to respect her own choice and religion if she is upset by the cheese.

It won't be the last time she will miss out on something if she is 100% kosher.

DP works long hours and says he doesn't have the time and energy to think about it

How much time is spending with DSD when she's with you? How would he manage his contact time if you weren't there to facilitate it?

He's being lazy defaulting to you but you do need to find a way for your own DCs diet and DSDs diet to align a little.

Could she have a non meat sandwich for lunch then meat for dinner?

What are her favourite/usual breakfast/lunch/dinner options? Can you give us some ideas so we can help?

I’d serve the same meal for all DC, so if DSD has beans on toast or fish fingers they all do.

The OP is cooking for at least 4 children for lunch and dinner for over a month. The OP is looking out for her very overweight DSD by not just taking the easy option the girl's father would and giving her a ready-meal. It's fair enough that the OP wants her children to eat things they are happy to eat over such a long period of time. The OP is also conscious that her DSD gets upset if she can't have cheese with her dinner when others are.
OP - your DP needs to follow the same diet as DSD while she is staying. He must follow all the rules she does. So he needs to take a more active role in this for sure. He also needs to not have the cheese on the bolognese etc. He needs to have the vegan sausages with the mashed potatoes made with veg oil not butter and so on.

I’d get meats from the kosher section of the supermarket, for the whole family, and serve that and any veggie stuff your DC will eat.

Ready meals aren’t all high calorie / salt and DC can be served different ones.

& as others say, sandwiches, raw veg, things on toast.

Would serve your DC their normal cheeses and DSD knows what her Mum’s religious food rules are and can decide whether to have it.

Would have zero regard for your H’s food preferences given his behaviour! If serving meals he dislikes or ready meals to the DC he can sort out his own food.

Shit parenting & sexist partnering by him.

Why should the step daughter "just have toast", and not the kids who make a fuss if they don't get meat in every meal?!
I'm saddened by the lack of tolerance for religious beliefs in this thread, although not particularly surprised.

I don't feel sorry for you because you're actively choosing to be the house slave that takes order from a man you're not even married to, and to pander to a child that's HIS, not yours. Then, after doing all of the slave work voluntarily, you decide to be a poor martyr about it. No.

Either happily be the slave, or stop pandering to this man. It's his child, it's his responsibility to look after her. Especially since he hasn't even thought you're worth marrying after everything you do. Not that I think marrying this useless idiot is a good idea...

I'm Jewish (but not Orthodox) and when with more observant friends we just serve blanket vegetarian. End of discussion, covers all bases. No need for separate kitchens etc.

However I strongly agree that this is mostly up to your DH to navigate and sort out, not yours, particularly if you have been doing all the heavy lifting before.

I’d just feed her what I was cooking for everyone else and if she can’t or won’t eat it then the husband can make her something else. Or she can have toast.
if she goes home crying to mummy that she was given things that she shouldn’t eat I’d just tell her oh well we aren’t Jewish here so we don’t follow that diet.
Honestly I wouldn’t even be doing childcare for her. If your husband wants his child to stay for a full month he takes the time off work and sorts her meals out. It’s not your responsibility

Bet you wouldn’t serve pork if your DC had Muslim friends round for tea?

Where or not your husband does his fair share of the family cooking is a separate issue.

Presuming you usually do all the cooking for a family, is it really a problem for you to avoid pork / pork products for a month and to not mix milk/meat so that you SD isn’t made to feel uncomfortable in her father’s home? There are dozens of perfectly simple kid friendly things you could cook without having to go out of your way to make something separate.

Pizza, pasta etc. with cheese and tomato sauce, fish fingers, chicken nuggets, bolognaise, jacket potatoes, stir fries, chili con carne, meatballs, roast chicken/lamb/beef, salmon, fish cakes etc. all with chips, potatoes, rice and veg, sandwiches, burgers, beef sausages.

Why would she be approaching anyone?! The child’s father can do it, instead of opting out most of the time