It depends on how much extra (if any) the UBI in question gives for having dependents.
The simplest thing to solve the problem (way more than finagling UBI rules) is just doing the opposite of what the Republicans are currently doing re: sex education, defunding of Planned Parenthood, and restrictions on abortion and birth control.
I am fundamentally opposed to a UBI. The question of how to solve the issue of paying women with children is just one detail that I believe is problematic. But I think it is fundamentally broken through and through and I hope to god, the devil and all points in between that we do not get one.
The simplest thing to solve the problem (way more than finagling UBI rules) is just doing the opposite of what the Republicans are currently doing re: sex education, defunding of Planned Parenthood, and restrictions on abortion and birth control.