Said no grandma ever
Apparently grandma thinks they are growing wider, rather than taller.
Im not native English speaker, so I cannot understand what does "a little too much" means.
It means "too much, but not by very much". Slightly over the line dividing "enough" from "too much". The kids aren't morbidly obese, but they should perhaps eat fewer/smaller snacks.
Why "The grandchildren" are incorrect here?
Because it says "mine" = "MY". You don't use the definite article in that case.