The Baby_Child cap won't do anything for migrant waves and migrant pregnancies. It's a matter of luck with migrants, if parents migrate to the fort they're going to drag their kids along. It's easy for parents in legends to have a dozen kids. Then you'll have a large family coming in taking up most of the migrant wave.
You may be able to get less children by lowering the maturity age for dwarves. If you can find the RAWs for dwarves, you set the [CHILD:12] token from 12 years to 2 years, 5 years, or whatever you want. Even if you do get migrant children, they won't take too long to grow up in this case.
If you set the child pop cap to 10:1000 then the limit on children will be 10 kids at a time, unless migrants show up. I don't know what the second number really means, it's supposed to be a percentage of the adults in the fortress. Don't know what that is supposed to do with the cap though.
Sounds like real life in an non technological society. Birth rates were high because the kids kept dying. Of course, if you want less children, you can always set the child population cap to 0:0.
That's half of it, but the infant mortality rate was high too as you said, so huge families should not be common. Or we would have seen massive population booms in the middle ages.