The Society for the Prevention of Childhood is exactly what it says it is. A loose network of people who have come to the conclusion that reproduction via children (as opposed to forking, AGIs, or other transhuman options) is unethical, immoral, impractical, or otherwise inadvisable. Although they have a cause in common, their reasoning is not always the same.
The Lost generation sits in the backs of everyone's minds, and some learn a more extreme lesson from that failure than others. One view is that reproduction is irresponsible and unethical during the infugee crisis. There are still tens of millions without bodies, so choosing to have physical children is taking biomass and exowomb time from them, and the only other options is to follow in the footsteps of the Lost. However much one might want a child, the needs of the existing outweigh the needs of the potential.
Another view is that the combination of children with transhuman tech is too dangerous. Some, especially older, established individuals, worry about the precociousness problem. Children engineered, raised, educated and well accustomed to post-singularity technology will likely surpass their parents, forming a generational singularity.
For a few members, childhood is itself bad. To them, childhood is inevitably a traumatizing experience and most people will spend decades getting past it. This view is usually held by people who have themselves had traumatic childhoods, often related to the Fall.
Neoprimitive members argue that humans are not adapted to the future, and children doubly so. Similar to the view that childhood is traumatic, and opposite from the view that our children will surpass and supplant us, some neoprimitives believe that childbirth is the cruel thrusting of a being into a hopelessly alien world. Neoprimitives are rare, and this view is rare among them, so only a few are members.