When I was a child, I remember sitting in some services with my parents. I wasn’t, of course, getting anything out of the sermon, but I was learning how to behave in church, and I still got to do things like find the hymn and sing along. It’s a valuable learning experience.
And as an adult, I really enjoy sitting near babies and their families. Granted, sometimes the babies are a bit distracting from the sermon, but the worship experience is richer as a result of them being in the service.