You probably have to give children some sort of amount. Maybe even the full amount. The public isn't going to buy into a program that gives an abled bodied man who smokes pot and plays WoW all day the full share but give a single mom the 4 kids the same amount.
There is also no way this would work if 50% of the country doesn't really get the income because you raise their taxes to take it back. Middle class taxes are a third rail in American politics. Even socialist Bernie Sanders won't raise their taxes.
You'd be better off just raising the earned income tax credit.
You're right I forgot to add back in some amount for children. Somewhere between $400 and $1300 would be fair, so let's say a middle ground of $850/month which adds back $0.75trillion.
As to raising taxes - if a UBI were ever to exist this would have to happen to the middle class, because the middle class (or most of them, might depend on how defined) do not need $2,600/month on top of their current situation and if they get this then certainly it wouldn't be affordable. It's not nearly as hard a sell as a normal tax raise though, you just explain that for X people under a certain wealth level the "new basic income tax" will be capped at the same amount they receive as basic income so that they don't lose any money in real terms.
There is also no way this would work if 50% of the country doesn't really get the income because you raise their taxes to take it back. Middle class taxes are a third rail in American politics. Even socialist Bernie Sanders won't raise their taxes.
You'd be better off just raising the earned income tax credit.