I do agree with what CM Mom said, however one of the only regrets I have is that I didn't put our kids together. I never dreamt it would be an issue since my kids happily run off in separate directions when I drop them off at school each day. My youngest had a complete melt down being left without his sibling and it took us lots of coaxing and work for two days to get him to go back.
I also thought it was really difficult to have them in different groups because they all follow a different schedule. I found myself constantly watching the time because I would have to drop someone off or go pick someone else up. Their activities don't even stop and start at the same time, it's staggered. They even ate at different times. To be honest, it kind of drove me crazy.
Just study the Navigators and decide what is best for your family.