We have a one-year-old, BH. Is it OK to give him milk and then meat and vice versa without waiting in between?
Many Rabbanim I have heard say that starting from 1 wait an hour, from 2 - 2 hrs, and then increase it to the full six hours by 6 - however as usual Ask your local Orthodox Rabbi.
However between milk and meat unless it is the type of cheese that needs waiting, even adults just need to take a drink and a cracker or wash out the mouth, etc.
Baby - 3 (no need to wait)
3 - Age of Chinuch (wait 1 hour)
Age of Chinuch (wait according to family minhag)