1) the subsidies are sufficient for both groups to do OK. This is fine in theory, but taxes would have to go up by a lot to have the minimum take care of the needy with the largest expenses. The UBI would have to be closer to 25,000 USD. If it's politically impossible to raise taxes to the level needed to let poor people live in dignity today, then what are the odds the American people sign off on the massive tax hikes needed to fund a non-destitution basic income?
2) the UBI simply isn't sufficient. Some people are getting money they don't need and other people are getting much less than they need just to survive. Every dollar that goes to the first group means there's less for those who really need it. In other words, the proposed system is a cruel dismantling of the welfare state.
Raising 3 children is practically a full time job, just one that happens to be unpaid. With the UBI the minimum wage would go away, meaning that the single mother is going to make a few dollars an hour in her part time job. That will barely cover the cost of the car she needs to get to her job, and the cost of the babysitter. The suggestion that being a single mother isn't much more expensive than a being a single young adult is absurd. I really struggle to respond to this. Just think about it: a single person can share an apartment and bike everywhere, enjoy the simple bohemian lifestyle. Being a beach bum doesn't cost anything. Do side jobs and hustle whenever you need a few bucks. The single mother in contrast needs a car and a house with rooms, just to start. That's going to take a big chunk out of the 10,000. Just living nearby a school is already expensive, and not optional. And the children themselves cost money too, in a million little ways. Diapers, clothing, entertainment, food, trips to the doctor, the list is endless.
I don't believe I'm misrepresenting the situation, this is just how the math works out. If the UBI means giving money to people who don't really need it then taxes must go up by a lot or the poor are getting a really raw deal.