Absolutely, your 12 can choose to be in the Lab with your 8 and 10 year old. As to allowing the 10 year old to move up....maybe. I wish
DCL would quit allowing this because a 10 year old in elementary school really doesn't fit with a 13 year old middle school kid, and the 2 year overlap should allow for most needs. The problem comes in when the older kids get frustrated with all the "too young" ones in the group, and then they don't want to participate.
Do realize that if your 10 and 12s split for the tween programming, that leaves the 8 year old alone? It seems more fair to allow the 12 to choose to be with her siblings or not rather than to place the 8 year old who will have no choice in the matter to be on her own. But that's a personal opinion, and YOU know your kids better than anyone else. You are the best one to decide who belongs where.