"poor people should have fewer children, rich people more"

This is stated as an imperative on the parents, not as a desired outcome on poor and rich children. State it as, "poor children should have fewer siblings, and rich children more". Doesn't that already feel better? It changes focus from the people controlled by the policy, to the presumed beneficiaries. It hides the gun in the room. How about "Poor children should have fewer siblings so they receive more resources from their parents, and rich children should have more siblings so they receive fewer resources from their parents." How warm and fluffy that sounds.

Applying Haidt's moral categories to the original: Harm, Fairness, InGroup, Authority, Purity, Autonomy.

Violates Fairness - poor parents are allowed fewer children. Pro Authority - sacrifice of individuals to social good. Violates Autonomy - increased control of both.

Given the usage of Rich vs. Poor, Rich is them, and Poor is Us. So this violates InGroup. Purity? Little effect, except for possible squeamishness about methods.

Four bads, one good.

From the perspective of the last phrasing - Harm - good, Fairness - good, Authority - good. InGroup - kind of a wash, because while the InGroup benefits, the InGroup gets smaller. If anything, I'd say InGroup - some bad, because size of the InGroup seems more important, somehow. It's the state of the Group, not the state of the individuals of the group, that seems to matter with InGroup. Purity - nothing. Autonomy - we've pretty effectively hidden the control of the parents, but there is till a whiff of it in the air. Autonomy - slight bad.

Three goods, some bad, and slight bad.