[Solved] How should we be feeding our kids?

I imagine there have been many threads started on this topic, but I couldn't see a current one.

I was wondering what everyone's opinion was when it came to feeding their child.

My daughter is generally good, but every few weeks or so when I have her at the weekend, she throws a fussy one. I'll make her lunch or dinner with things I know she likes, or at least eats, and she'll tell me she doesn't like that food anymore. It can be very frustrating when I have just spent 20 mins in the kitchen preparing a decent meal for her. She's 3, soon to be 4.

I try to encourage her to eat as much as of what's on the plate as possible (but making sure she lets me know if she's full), and if there's something new on the plate, to at least try it. Both of these things count towards her moving up her behaviour/reward chart, which she takes great pride in. Though I didn't feel I was being overly strict with this approach, yesterday, having only had a few mouthfuls, she refused to eat any more, and wanted her yogurht. She said she was full, As best as I could with a 3 year old, I tried to explain that if she's full, she wouldn't be able to have the yogurht because there would be no room in her stomach for it. She was obviously not full but trying it on. I encouraged her to eat some more of her dinner if she wanted her yogurht.. She started crying, saying she missed mummy, which she says often when I am disciplining for whatever situation (I get the strong impression from what I know of my ex, how her mum let her run wild as a kid, what my daughter has told me, and how my ex is around my daughter when I've dropped her off, etc, that mummy is a soft touch). I asked her if she missed mummy because she didn't want to eat the food that daddy had put out for her, to which she said she did. It made me feel rubbish, and that I was forcing her to eat, but at the same time I feel that I wasn't, and that I was just being firm with my parenting. I don't want her to feel that she can just say no to food and then she will get pudding, after all. At the same time though, I keep seeing stuff online about how parents should leave kids to it with food. In the end we agreed that she would eat a certain amount of what was left on the plate, and I separated x amount of mouthfuls for her to eat, and she was happy with that.

There must be a happy medium here somewhere, and though I thought I had found it, I am now wondering if I haven't, and whether I am being too strict about eating. I am always worried that I will send her home hungry,

On the flip side, my ex and her mum overfeed her. Just last weekend, I picked my daughter up from her granny's, with a stomach ache for the umpteenth time due to granny's huge breakfasts, in which she she feeds my daughter porridge, and then continues to ask if she wants more, to which my 3 year old obviously says yes, and is then fed toast, sandwiches, various meats, fruit, and more, until her stomach feels uncomfortable (my daughter doesn't know how to say 'no' to granny it turns out, having spoken to her about it), and all of this before 8am. I am confident my daughter isn't exaggerating with this, as she has told me this a number of times, but also when I have picked her up from other places and she has told me what she eaten, she never says she has eaten this amount of food (apart from with my ex, but at least she stops at porridge, fruit, and toast).

Or am I the one in the wrong here? When my daughter stays, is a single bowl of porridge, weetabix, or cereal an unacceptably small amount for a child to eat for breakfast? I only ate the one bowl of cereal when I was a kid. I still do now. And I've never keeled over from malnutrition.

All a bit of a pickle. Would love to hear people's thoughts on the matter.