I had my fourth when my oldest two were in elementary and third was in preK, DD7.5, DD5.5, and DS3.5. Once the baby was able to sit up, they began to include him in their activities.
My favorite is when they include him in their games of "school," where they are the teachers and their baby dolls are their students. I was certain they would make the baby one of the students, but they informed me that he is "staff" and that he teaches the subjects of crying and sitting.