All hours spent with kids under 5 is basically 'on'. Which would leave most mothers I've met at zero 'on hours' left for work. A reason for the 'gender pay gap'?
Childcare is skilled, necessary labor, and should be subsidized by the government at the level that food production is. Maternity leave and paternity leave in the United States are also woefully insufficient.
Proposal rejected by society. You will raise the child into a productive citizen, then society will tax the shit out of them while yelling "we paid our property taxes for our shit schools, so we deserve a slice."
Then the social security cut gets distributed regardless of preference to who raised the kids who now pay the taxes.
It's a classic tragedy of the commons, offload almost all the cost on moms then socialize the taxed gains.