V.B.
Our Sunday School is during the Adult Bible Study hour prior to the service with Holy Communion. During the worship service, everyone is in attendance. Most larger churches have a cry room, so that moms (or dads) can take babies out if need be, but the other kidddos pretty much learn to behave.
I am a logical person. Think about it. If your kids ages 4-7 are never required to sit and be quiet for a full service, how do you think they will learn this skill? And why can't a 7 year old do this? They do it at school for longer than that, don't they? I think it is more the perception of today's generation that kids "are kids" and should not be held to any sort of standard... let them be what they are. Well... if we want to mold them into productive members of society, we have to teach them how to behave in certain settings.
My husband used to have to work Sundays, so I had the kids on my own going to church every week. My daughter was 3 1/2, and my son was 7 1/2. They sat with me, just the 3 of us. No extra help. No leaving part way through. They learned to follow along with the service. When to stand, when to pray, the various parts of the liturgy, etc.
My kids are not "special" nor are they little angels. But they knew what was expected of them, and I reminded them before the start of the service each week. We sat down front, so they could see better, and I think it also made them a little more conscious of other people watching/seeing them if they misbehaved. They didn't take toys in, either. I'd let my older child use a pen to write in the bulletin if he wanted. Otherwise, he was expected to participate in the service, the same as me, and my daughter was learning the ropes, too. She was an early reader, so by 4, she was belting out hymns along with everyone else. She'd often earn a smile from the Pastor and the Elder who could clearly hear her on every hymn.
Unless your child has ADHD-hyperactive, and is unmedicated and it is uncontrolled, then there really isn't a reason he cannot be taught appropriate behavior.
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ETA:
Also, wanted to point out: I can't speak for all denominations, but I am Lutheran, and in our services, the longest a child has to "sit and be quiet/still" is about 20 minutes at one time (during the sermon). The rest of the service is smattered with bits of activity. Standing for hymns, prayer, responsive chanting/singing, Confession/Absolution, the offering, Holy Communion, etc. The 2nd longest chunk of time of "sitting and being quiet" is during scripture readings, and even that, we rise for the Gospel reading, and there is liturgical responsive singing in between each section.
If you have your child actually participate (expect them to, not ignore them crawling around in the pews and on the floor, turned the wrong way looking at people instead of paying attention to the service), not only do they learn the service (which I find beautiful), but they are busy! It's not at all like going into a lecture hall and sitting like a bump on a log for 60 minutes without being able to so much as clear your throat. And by the time your boys are around age 9, they are likely to be really participating (as in being an acolyte, lighting the altar candles before and putting them out after the service). My son began doing that (and sitting in the chancel during the service along with the Elder and Crucifer and Pastor) when he was in about 4th grade. Now, he is Crucifer (at 14).
Bringing the kids in at the last 15 minutes deprives them of understanding what is going on! And when they miss the beginning of the service, what motivation do they then have to care what they have essentially walked into the middle of?