Like Peacefulbird, I stay with the children until all asleep. Where mine are divided into 2 rooms, I sit in hallway outside the two rooms until I know all are properly asleep. As said, here, that's barely a couple mins because I make sure all children are properly worn out and ready to nap.
For me, I have always divided the two ages simply because unlike some day homes, I don't force an awake child to remain in the nap room once they wake. Some children really do need a shorter period of rest then others. By dividing the ages, I am not waking up the smaller children when I take a waking older child from the nap rooms. A three year old doesn't need to rest as long as a 10 month old. You will find what works for you too and part of that will be the rooms available to use.