It’s unlikely that it’d be $20k per person. Probably in the $10k range + like $3k per kid or something. Your second paragraph is only true if they print money to do it. Otherwise you are not creating net inflation (although certain products will go up and others down as resources are re-allocated). Most UBI proponents also propose eliminate all other forms of welfare (of which there are too many to list) and it benefits middle class workers and not just the lazy and poor. It’s definitely not needed today, but far down the road if robots and automation really do replace 75% of today’s job, it very well may be needed.
Per kid? Since they’re so against overpopulation (pro-abortion), they’ll limit increases to 1 kid.