The church that I grew up in had multiple (2? 3?) services every Sunday. One of the services was scheduled at the same time as the Sunday school classes; another frequently (but not very reliably, to my memory - I always considered it a pleasant surprise, though I suspect it was announced ahead of time somewhere) had the children sit through the first half of the regular service, then go to the front for a short 'children's sermon' followed by an activity elsewhere in the building that was supposedly a kids' version of the regular service after that point. (This was an optional thing: Kids were invited to go up for the children's sermon, but could opt to stay with their parents and sit through the rest of the service.) There was also a nursery/babysitting service for the little kids during all except maybe the earliest service; this was staffed by volunteers, but teens in the year leading up to full membership in the congregation were expected to volunteer a certain number of times for that or handful of other roles in order to be accepted. It didn't seem to be especially unusual for parents to bring their toddlers to regular services, but if a toddler became disruptive the norm was for the parent to drop em off in the nursery. It definitely wasn't normal for kids to be running around loose on the church grounds during any service, though. After, during the social period, yeah, but during the services we were expected to be at the service or in an activity of some sort.

The Unitarian church that my parents took me too (both of them, actually, the one in Washington State and the one in Ottawa, Canada) had a Sunday school, with classes divided by age. I remember reading a storybook about Buddhism, coloring, acting out stories from the Old Testament, etc. There was a class for slightly older children (I was maybe 11) and that was where I got my first book about puberty, and the grownup talk that came with it. I don't remember finding Sunday school incredibly fascinating, but I don't remember not wanting to go, either. (My brother hated it and his resistance was the main reason why we eventually stopped going, but he also hated skiing lessons, cottage trips, and pretty much all the activities we did as a family.)

At the Catholic churches in my area (the ones I am most familiar with since I have to 'volunteer' for said activities), kids are only expected to sit through the boring parts of Mass once they hit middle school.

Before that, they're taken out or downstairs for the 'Children's Mass' (don't know the real name), which is more arts & crafts combined with colorful pamphlets and story telling. In summers, a lot of them attend 'Vacation Bible School' which is the Children's Mass writ large and with outdoors activities. Teenagers get to go to 'Youth Ministry' events, which are heavy on hanging out, music, and that sort of thing (the most loyal getting to go to a big festival thing in Massachusetts or somewhere) - the events are a lot more popular than one might expect.

Judging from the videos produced by the diocese, they seem very pleased with the programs' success with young people and retention.

One synagogue works like this: Children can either sit next to their mother or father and pray, or stare off into space, or amuse themselves quietly with books or toys; or they can wander around the hallways and dining rooms and lounges and playrooms and play with each other. By the time they are teenagers, they are expected to participate in the ritual as adults. If a family has a baby, one parent (usually the mother) will arrive an hour before the ritual ends with the baby, and periodically carry the baby outside to keep it happy. They can also keep an eye on the older children when they do this.

Another synagogue has all that, plus a structured activity for schoolchildren and teenagers, led by a lay clergy.

The above two paragraphs are also accurate descriptions of family dinners in my family. Instead of praying there is eating.

In all the Christian churches I have been to, the children are expected to sit in the pews the whole time. I suppose this is practical if the ritual isn't too long, and desirable if they can get some value out of the ritual.