Under Voldemort's reign of terror only the bravest or the wealthiest had children. The unknown exchange the Dark Lord made for his grab at power and immortality was the impairment to the continuation of wizard-kind. Unable to marry muggles and muggle-borns, witches and wizards saw a drastic decline in population that could not be stalled. Whether due to sterility caused by too much inbreeding between the pureblood families or simply fear of what would happen to any child born in this era of darkness, there were simply no children. What would they be learning? They would have no memories of a brighter world and would not question the evil power that enveloped them.
The wealthy could buy some freedoms for their offspring, but it was risky. The young ask too many questions that the old know are dangerous to answer. A cry or a laugh at the wrong moment could mean death no matter your status and it is too much to ask babes in arms to be so stifled.
There is always equal exchange and you don't know what you're giving up, but such trivialities matter not to someone like Voldemort for whom domination is everything. If there are less to dominate then victory is simply closer.
Pockets persist, but they are well hidden and those children know a different kind of darkness; one of shadows and secrecy.
When Harry Potter arrived at Hogwarts he had no idea why his class seemed so small. The dormitories had been adjusted to hide the empty spaces, but there was a deep sadness in the professors' eyes when the new first years arrived that went unnoticed by the young pupils dwarfed by the grandeur of the great hall. There were so few.