Interesting, since I have the opposite problem, i.e. very few children in total. Most of this is from 0.40.19, but something like 4 fortresses had a total of probably 6 children born and one migrant over a 10 or so year period. In one fortress I gave the two migrant coupled (both fertile according to DT) 3 months off, but no children a year later. In the next one I resorted to "mating", i.e. burrow the 3 migrant couples I had into their rooms until I saw them actually side-by-side, and I still only got one child, and that one was early, i.e. conceived before my cattle handling of them. The next year, same thing, 1 out of three (different couple).
The only fiddling related to this I've done is to reduce pop cap to 50 and absolute pop cap to 80, leaving the child ratios unchanged.
On 0.40.13 I DID have the massive child wave issue. My current 0.40.23 fortress is only 1½ years old and has received only one couple, but no children have been produced yet.
Peasants are not useless, if they've got potential, since training is typically fairly quick. I'd certainly take a peasant with potential for useful skills than another master wax worker or potter (although given how powerful glass making is, a potter would probably pull his weight if you've got clay and magma) any day. If they don't have any potential and aren't liable to be subjected to babymashing, you've got some militia recruits or haulers...
The description of the percentage indicates that you could have 10 children to each adult as a maximum, provided the other cap isn't hit first, so lowering that ought to have a reducing effect.