Without making myself feel really old, £20 p/w is a lot. I was promoted to an allowance of £20 p/m at 13.
But really..
Do they do a rota or have allocated job to contribute to the house to earn that money? Even if it’s just mowing the lawn, emptying the bins, etc.
What do they have to do with that money? DO you buy them casual/school clothes so this is all spending money?
What do you want them to do with it? The cinema is a lot more expensive than it used to be! My £20 p/m would have taken me to the cinema 5 times. Do they have to fund their hobbies out of it?
Can they supplement their income? I did a paper round from 13-16 then worked in a chippie. Do they/will they have that option?
So basically, comparing to a national average is a very simple yardstick.