Your right parents don't get extra tax free allowance for kids. But they are also financially responsible for their kids regardless of where they live.
A simple way round taxing maintenance would be instead of Dad paying the money to Mum he pays it to the kids, utilising THEIR tax free allowance, into an account that Mum is in control off, as obviously a 3 year old isn't really in control of the shopping bill.
Which is fine when the kids 3, not so easy at 15, but Mum thats MY money, why should I pay the shopping bill? As the maintenance money gets frittered away on junk.
Taxing maintenance is unnecessarily making a complex situation even more complicated.
And leaves Mum at the mercy of her ex or potentially children.
Then think about Dad giving his kid a Christmas gift of £100, is that income to the Mums household, should Mum be paying the 20% tax on it?
Actually thats another way to abuse Mum, give 100s to the kids that Mum then needs to find the money to cover the tax bill.
Dad gives 15 yo £100 to buy trainers, is that £100 classed as Mums income? Or only £50 of it as the trainers will move with the kid between households?
I actually can't believe anyone thinks its a good idea.