My kids used to thrash a bottle of squash in a day. Mostly filling the cup with too much and adding a hint of water! So DH and I bought a keg type 5 litre jug. Each morning I make up the squash in the jug and the kids can dispense their squash as they please. No more bottles gone in a day.
I've also had problems with sweet stuff being taken. So DH and I decided that we would try a different system. The children can have something sweet after dinner, if they have ate it all. If they haven't, that's fine, they can have fruit. It's not to make fruit seem second rate, if they haven't ate a lot of their dinner, there is a good chance the veggies won't have been ate, so having fruit is better than something sweet. It works really well thankfully.
I've noticed my ds who is 9 years old and tall for his age, has a much larger appetite than he used to. He also has a tendency to comfort eat and will try to find something sweet if his mood is low. Whilst it doesn't make it okay that he will help himself, it does diffuse the irritation about it. It's like a compulsion.
All my children are slim as am I and my DH, all the children have great teeth thankfully.
I really don't like referring to a child taking food as stealing. I know technically that's what it is, it just makes me uncomfortable. When they are in their own home, they are being dishonest to take treats without asking, but I dont feel it's stealing. I grew up with 3 brothers and all four of us used to punch biscuits out of the biscuit tin. I remember pinching a chocolate digestive and making it last for ages, just nibbling tiny bits and letting it melt in my mouth. Happy times! I can say that every time I took something, I was actually hungry. I had a very fast metabolism.
Not every child is fine with 3 meals a day. As an adult, I need to eat 6 small meals a day due to Gilbert's syndrome which I was born with so possibly why I couldn't go so long doing three meals a day.
Some children and adults also have hypoglycemia, so not diabetes but are affected by low blood sugar levels. I have this and as a child, any time I did something energetic, let's say swimming, I would end up feeling weak and shaky afterward and had to have something to eat. As I now eat 6 meals a day, I don't get it very often anymore.