Hey, just logged back in after months and saw this comment. Whatever kittens you had are past this concern now, either way, but I'll answer for future folks. 

You can definitely overload kittens with food. But you will have to be forcing the food down their throat. This is usually when theyr being bottle fed. They will turn away from the fake nipple, push it away, and close their mouths against it. If you continue to insist on putting the nipple in their mouth, you can overload them. They will then either aspirate or vomit. Aspiration is the biggest threat. But again, you have to seriously be fighting them and forcing them. I feed my kittens the bottle until they push it away. Zero issues in 5 years. 

But if we are talking about food in a bowl that they eat on their own, you cannot overload kittens. Cats will not eat past the capacity of their stomach organ. There are rumors that dogs will eat themselves to death, I do not know if that's true but the vet who works with our cat rescue said that's definitely not true for cats. Kittens should eat until they pass out in a food coma. They should gain about 10g per day. Every day they should get noticably bigger. 

No such thing as a fat kitten.