I've been doing 90% off my Christmas shopping online this year and when I was looking for cashback I discovered KidStart, they have tons of big shops on cashback. I was surprised to see Amazon and John Lewis because they aren't normally on cashback sites and the cashback you make can be paid into your kids bank accounts. I thought it might be a nice way to top up their bank accounts?
Anyway, I made a few pounds back already and some shops have discount codes too, hope it helps if you are still shopping!