Most proponents of UBI see it as, at least eventually, replacing some range of existing government means- and/or behavior-tested benefit programs and, possibly, some age- or disability-tested ones (in the US, most advocates would probably include, at a minimum, EITC, General Assistance programs, SNAP, TANF, Section 8 housing subsidies; some would also include Child Tax Credit, Tuition and Student Loan related credits and deductions, Medicaid, Medicare, and/or Social Security.)
I've never heard any advocate argue that it would replace all social services.
Personally, I'd like to see us end the welfare programs for the rich.
What welfare programs for the rich? The rich pay for everything. The bottom 60% net a negative ten thousand dollars. The next 20% break even (~$1000/year paid), and the top 20% pay out the nose.
ETA: Go read the report for yourselves: https://www.cbo.gov/publication/49440
Anyway, if people really are proposing that the state should give money away because then there is no need to protect children from bad parents, that's pretty stupid.
If children are not being cared for, the state will still intervene.