Kindergeld is a fantastic example of something that should be a UBI. It's intended to go to the child and be spent by the parents on the child's behalf - legally, it's considered income of the child, not the parents. That's literally a basic income already and it's paid from birth until the child is 18-25 years old (depending on employment status or if the child is in education).

Of course because the FDP conveniently forgot that its "Bürgergeld" was supposed to be a form of UBI and decided to simply rebrand social welfare (aka Hartz IV) instead, we're now in a perverse situation where the thing that was supposed to be a universal basic income is instead very much not universal and even takes Kindergeld into account if the child is part of the household.

That said, turning the Kindergeld into a UBI would also mean abolishing the Kinderfreibetrag which acts regressive (i.e. in effect, higher income families receive more benefits per child than lower income families). It would also make it easier to justify closing the loophole of migrant workers collecting child benefits for children in their home country given that a UBI could reasonably be restricted to national residents rather than all EU residents (which is the concern with limiting child benefits the same way).