We support children for years and all we expect them to do is go to school. While a lot of kids could decide to just not work and be taken care of by their family, the vast majority want to work anyway, if only for the fact they want to move out and build their own family.
This may be the key. UBI + Making people go through further education so that they can have the foundation of knowledge and skills to be able to work. Right now, we have a lot of people who go through years of school and in the end it amounts to nothing because no one will give them a real job unless they had the money and a stable environment that allowed them to go to fancy universities instead of being forced to work to provide for themselves. This is a 3 angle attack. UBI + education/training + lowering the barrier for people being able to get a job. With UBI, you will have substantially less people desperate to get any job, so the supply of workers is no longer so high that companies can afford to be picky, because people will be able to tell those companies to go fuck themselves.
Society benefits as a whole when we foster the development of proper adults. If the development to making them a net contributor is not done yet, why would we stop?