Also I think that there might be more children from older wizards, but they would only mention those with magical talent. All the others would have died long ago. Some wizards look out for their descendants without them knowing, some might take an active role as the old great-uncle or aunt. But I can imagine that most old wizards have lots of human descendants.
And I can understand why they might only take notice of the wizard level ones.
It would be to protect themselves from living through the aging and the deaths of dozens or hundreds of children and grandchildren. So you would do better to keep yourself sane, if you didn't attach yourself to those of your children who won't live a wizard's lifespan.
I think this is what McCoy was doing all the time. Who knows how many non magical kids he had before Margaret LeFay. I can't remember if it was in one of the short stories or in one of the microfictions or even in Peace Talks (I think it was in PT during Harry's and Eb's talk in the svartslf embassy apartment), but I got the strong impression that Eb was used to the tradition of giving away your child to be raised far away from you without knowledge that magic even exists. And to watch and wait, if the child develops magical talent. If it doesn't, the parent turns away and leaves the child to grow old. But if it does, the wizard-parent would make sure, the child is properly educated in the magic, either by themself or by another adequate teacher.
McCoy did that with his daughter and he did it with Harry. And maybe even Harry's Mother would have left him with Malcolm, had she survived, until bis magic kicked in, because it is tradition to do it that way.
And thinking about it, I could imagine that it would probably be the male wizards who have more children, because as a female wizard I would not think it funny to go through 20 or more pregnancies and births in my life. And I would not be able to just leave the kids with their fathers, once I have given birth to them. A male wizard can just make love with a woman and leave her pregnant. Child care problem solved automatically.