I don't think it's possible to be strict too soon. My son was a great eater intil about 18 months when he just stopped. I had over a year of problems, and now he eats fine again. Some days he would only eat 1 rice cake, or half a weetabix. The only things I could rely on for him to eat were rice cakes, dry bread weetabix & plain yoghurt.
I got lots of advice from hv, doc etc & the common threads (& I can testify that they are true & work) :-
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Don't get stressed about it, or show you're cross/stressed, you must be nonchalant whether they eat 'well' or 'badly' - you don't want food to be an issue or weapon.
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As judetheobscure says, fixed times for meals & snacks is important. I live in Switzerland and even at mum&toddler groups they have fixed snack time, eg 4 pm, when play stops, everyone sits at a table and eats a healthy snack - eg slices of apple, lumps of cheese & some crackers.
If ds didn't eat I would gently point out that he wouldn't have anything to eat again until whatever the next meal/snack time was. It is important you stick to this.
- Never offer an alternative. And while food intake is minimal only offer healthy food, so you know he isn't filling up on food of little or no nutritional value. Now I will happily give my boys sweets and treats (not too often or they want them all the time) after all, there's only so much time you can scoff the biccies behind the fridge door before you're discovered.
Those were the 3 cardinal 'rules' . While I never even contemplated providing alternative meals, I did get cross on many occasions, mainly because I was fed up at all the wasted time & food. I also allowed him to eat , say half an hour later if he suddenly decided he was hungry. I think this was a mistake.
As soon as I took a million steps back (sometimes I just had to leave him at the table with the food while I bustled around on the other side of the door so he couldn't see me, he'd eat a lot better.
But at the end of the day, he genuinely often didn't want to eat & needn't maybe need to - my doc reassured my (many a time) that toddlers don't starve themselves.
Sorry to go on too long, I've just read your last sentence again, and I do maybe think it's a mistake to say something along the lines of 'Well, you won't get anything else unless you eat this', firstly because it's making an issue of it & will draw the battle lines, so to speak, but also I don't think you should get a child to eat one food just in order to get another. Do you see what I mean? It makes the first food into the 'badie' (and that's usually the healthy good food isn't it?) and the second food highly desirable. I don't think a child should be forced to clear their pate - as a parent we can decide on what's been a reasonable amount, and if they've eaten an acceptable amount we can then give them the next course (why doesn't that look right?) unlinked to the 1st. But that's just my opinion.
Good luck and don't stress, just stick to your guns & be consistent.