Furthermore, I have read up on the history of the American welfare program. It was dreamed up as a means to "help poor, single moms" at a time when intentionally getting pregnant out of wedlock was essentially unthinkable, so most poor, single moms were widows, not women who had given birth out of wedlock. The design of the program fundamentally changed the social contract and actively promoted the numbers of poor, single moms in the U.S., in part by making it less stigmatizing to have kids out of wedlock.
So, whether it is intended or not, if you pass a UBI, an awful lot of people will feel like deadbeat dads are not such a big deal and you will further change the social contract. The indicators I am seeing is that many of those changes will be negative changes rooted in a presumed lack of responsibility for our actions.
Welfare has the same problem with creating deadbeats but much worse, because if the father does actually pay child support or for that matter stays with the mother to raise the child, then the mother loses the welfare benefits. When you have two parents who are very poor but love their child it creates the unconscionable incentive for the father to leave so that the mother then qualifies for welfare while the father sends what money he can in cash under the table.
UBI at least improves the status quo by eliminating that.
Both child support and alimony are not flat basic-necessity calculations, but are income based. The availability of a UBI doesn't change the fundamental premises on which those are built, or even make any significant impact on the formulas -- under the assumption that a UBI paid to the adults would be counted as each adult's income, and UBI paid for the child to the custodial parent(s) would be treated as joint income when the parents are together and income solely of the custodial parent in the formulas after a split, there would be no need to even touch existing formulas.