First of all, children's church is NOT for 'sending those unruly brats' someplace else. If the pastor for the regular service is preaching in such a way as to challenge the Christian adults, or educate them, then the sermon will be, for the most part, WAY above the heads of most kids.
It is important to remember, too, that when the children were in the crowds listening to Jesus, they were outside, where they could sit down, stand up, pick flowers, wander around a bit, etc. Not exactly the same as church! Nor do most ministers have the same appeal and ability to communicate that Jesus did...
But perhaps children's church should be optional for the family, too. If a family wants their child or children to remain, then that should be fine. In addition, most of the churches we have visited and attended which have children's church keep the kids with the parents until right before the sermon, so they ARE worshiping with their parents, and learning the hymns and worship songs.
The age at which children's church should be stopped and the youngsters kept with their parents the entire time is a matter of both the parent's choices and the preaching style of the pastor. I can remember being more interested in the pastor's sermons when I was in the fourth and fifth grade than I have been in some in some churches in the past decade! If the preacher does not have the ability to speak in such a way that the children can maintain interest this does NOT mean he is a bad pastor! Everyone has separate skills. But the kids should be allowed to listen to someone who does communicate the same material and lesson on their level.
Then encourage the parents and kids to talk about what they heard on the way home or at Sunday dinner. Both will learn. Sometimes, seeing things through a child's eyes can be quite revealing!
Some churches have crayons and papers for the little ones during service when there is no children's church. This is a waste of crayons and the kids' time in the interests of 'keeping the family together.' The kids are sure not there mentally!
It is not lazy parents which begat children's church, but parents who 1) wanted time to worship and learn quietly and 2) wanted the same for their children. Remember, it's those 'lazy' parents who fund children's church and, in the case of the church in our old home where I was the education director (fancy name for scheduling and ordering materials and holding meetings for the pastor and teachers at our home!), it was the parents who did the most work, helping teach and as aides during children's church -- taking turns so other parents could have that quiet time with other Christians, worshiping and learning.
It is not lazy to consider your children worth a little extra effort or to understand that they will need a few years on them before they can learn in the same way adults do.
Age? Children's church is definitely beneficial up until grades 2 or 3, even 4, with an 'iffy' area, depending on the style of the pastor, up until about junior high.