The best idea I’ve heard for how to implement a one child policy is to mobilize grandmothers. Grandmothers are past child-bearing age and no longer feel the influence of their genes to have children. Instead they are concerned about the survival of their grandchildren.
If we could educate grandmothers on the imminent threat to their grandchildren by human overshoot then it might be possible to mobilize grandmothers as a single issue voting block in favor of a one child policy. Grandmothers are a large enough group that they could probably sway most votes if they voted as a block.
There is some history to support this idea. The prohibition of alcohol in the US, which required a majority of states to approve a change to the constitution, was spearheaded by a small group of dedicated women fed up with widespread alcoholism in society.
The threat from human overshoot far exceeds that from alcohol abuse so perhaps this idea is not so crazy after all.
It’s been many years but I think I heard this idea in a podcast with Jack Alpert.