My brother's idea was similar, but suggested negative tax for the first earnings, then with an increasing scale. No other benefits.

Not working you get paid as your tax is negative. As you start to work this decreases slowly, so there is still an incentive for working.

As an adult, you are expected to budget for children - or not. No extra funds as if you can't afford them, don't have them.

Very simple to run and understand. Just insert earnings and taxes / benefits pop out. Very difficult to abuse as there isn't much there to abuse.

The need for masses of civil servants reviewing forms,, printing forms etc will be removed. The need for so many accountants to fight through labyrinthine forms will decrease. And so, taxes overall can also fall as there is less overhead, and probably less acidental overpaying / fraud.