As soon as my weaners are eating solid foods and can reach the water bottle, mine are separated out. Especially if more females are about to give birth.
Babies, even after supposedly being weaned, will crowd out the newborns for milk if given the chance. Which can be detrimental to newborns. Not only does it risk smothering, but the lack of nutrition is obvious.
Plus, I've had a few females kill their newborns because the still nursing older ones were stressing them out beyond belief.
If you have the room and they're capable of caring for themselves adequately, separate them into their own container. If you're holding any back to be future breeders for the colony they were born in, you can leave them in if you desire, but I usually try for no more than 2-3 at a time.
Most of the time when separating out, those mice are being raised up for larger snake food anyway. So if they breed before being fed off, its of no consequence to me. I usually leave both sexes together to save on room.
Hope that answers your question. =)