AIBU to take my son's phone after he stole £20?

Have you spoken to him? Seems odd to go straight to punishment when you haven’t.

I refer to it as Choose Your Own Adventure. A child who admits it, apologises, offers to make it up, has a reason (bullying) gets very different consequences than a child who doesn’t.

Oh and chores aren’t linked to money. DD gets her age in pounds a fortnight and does chores because that’s what families do for each other. Linking chores and money IME makes it firstly a choice, and secondly a battle.

Have you spoken to your DS yet? I think this is where you draw the line about picking up bits of cash about the place and you tell him he needs to repay it pronto or to do chores in order to repay it because it's stealing. And from now on he earns pocket money via doing chores. You do need to check he's not being bullied or using the money for something criminal.

I think I'm with you that its unfair to punish him when he didn't know you'd be bothered by £20 in one go but not a cumulative £20.

I'd go with sitting him down calmly, setting the expectation from now on and agreeing a way to recover the £20 e.g. by not getting the £4pw until its repaid or chores to that value.

As a bigger point for not raising a lazy sod, I think I'd also stop giving money for nothing as it sends the wrong message, which is thst there is a basic level of expectation to participate in being part of a family (we are all responsible for the running of a clean home)

Is there something about the journey to and from school that makes it perilous enough to need a phone?

Why have you not nipped this in the bud previously?