You only need kids if your society is in a position where those with the wealth to afford children are not having them. Japan certainly has that problem, currently, the US does not. If you are in the Japanese situation, I would still argue opening up immigration is almost certainly cheaper an option than subsidizing child rearing, but we are no where near any problems stateside involving population replacement, and if we did, again, we can open our borders much more before turning to child subsidy.
If you are going to replace present means-tested benefit programs and not adversely affect current recipients, you need to have a lot higher per-person level of UBI if children aren't covered beneficiaries.
The most sensible target population I see for a UBI in the US is probably "all US citizens and legal permanent residents (green card holders)", without respect to age.
> and unless we need kids
We do. Permanently greying population would be a serious problem.