Enforce a rest period. Give the oldest a nap bag with quiet activities, a quiet place (away from you) where they must stay until rest period is over.

I would enforce a mandatory lie down for the others. At least 30 mins, if they are still up after that, quiet activities. If they don't stay quiet, back to the mat.

This is what I do with my group. Kids don't have to sleep, but they do have to rest. A daycare day is long and their bodies need that recharge time. I call it 'rest' time with them and we talk about getting a good rest so we have lots of energy to play after. They will lie quietly for 1 hr with quiet activities, then they can get up and play quietly until my nappers are awake.

You need a recharge in there, too. I actually prefer older kids. My group right now is all btwn 2-3 and I love it.