I thing that even when one is not yet chayav in chinuch, (which could kick in at age 3, 5, 7 depending on which mitzvah) and don’t have to stop a kid from being over an issur, you still can’t actively make him do an issur. So if I am correct then I don’t think you can serve him milk at a meat meal.
In terms of waiting, my husband asked s/o and was told (again, I think don’t take this as halacha) in the begining not at all just have a drink, something to eat in between, whe they are a bit older an hour and then at a certain age to wait the whole 6. I can’t remember the ages.