It's an antiquated system, and clearly won't work when individuals are assessed separately. We cannot go back to assessing householRAB, so it has to change.
My suggestion is to split it between the PAYE tax and the benefits systems.
Firstly, cancel the child benefit system as it is, then use as a basis the money that the government would still want to spend on it, £x bn say.
Then make sure that there is enough added to basic benefits to compensate those people who receive it and who don't pay income tax (which would be most, I guess). Take this from x, to leave say £y bn left over.
Now add an extra 20% tax allowance to all those currently receiving child benefit, so that the total extra giveaway is £y bn. In other worRAB, no matter how much your income, you only gain tax relief at 20%. It's possible to do that: Brown did it a couple of times in his day.