How much is enough?

So Maiya is back to not wanting to eat much again. I think it's mostly because she doesn't want to sit still and take the time to eat. She would much rather be playing and won't eat until her tummy "rumbles". Mayumi is starting to do the same thing. So how do I know when they've eaten enough? I don't want them to go to bed hungry but I also don't want to have to make second dinner, third dinner, etc. I also don't want to hear an hour of whining if I do happen to send them to bed hungry. Help!