No help with advice on what to say but mixing SA dcks and younger kids was always so hard.
Older kids are rough, don't pay a lot of attention to where the littles are in relation to them so accidents seem to happen more often, older ones can be downright sneaky, "I'm so boorrred", crude/rude, etc., etc. A Lot of them anyways. I had a few that were great helpers and loved the little ones to pieces!
Looking back I believe that was probably 1 of the top 3 most difficult things of running a daycare, trying to keep all ages.
You've got to do what's best for you and your dcks, not to mention your own sanity.