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.