I've got one child in kindergarten and another one that will begin there next year. The way it works is that the youngest ones have a group for themselves, with ages from 2½ to around 3½. From 3½ until school age, they're all in mixed groups, what they call "family groups".
When your second child starts kindergarten, he'll go into a young group first, but then we are facing a choice between letting him stay in the young group until he is the oldest, or putting him into a family group sooner than that. Based on the experiences with the first child, it's not fun to be the oldest in a young group, so "graduating" sooner to a family group seems preferable.
Now, the question:
There is more than one family group. Should we put our second child into the same group where his older brother already is, or should we put him into a separate group? What are the pros and cons of each alternative?