Dd is nearly 15. A few years ago she had quite a bit of money from an event plus money from relatives, birthday money etc. She used some of it to buy a laptop and a few other bits. She agreed she would save the rest and I opened a separate savings account for her where, at the time, she was earning a bit of interest.
Then interest rates dropped so I shopped around to get her a better rate. I opened a Junior Cash ISA which is paying 2.25%. However the money is locked in until she is 18.
She's now very upset that she can't access the money. She wanted some expensive trainers for Christmas, I said I would pay up to £X and she would fund the rest. However she thought she would take it out her savings and I thought it would come out of her monthly allowance from us (£50/month).
I opened the ISA partly because of the interest, and partly because I didn't want her frittering away her savings on clothes.
She says it's her money and for her to decide.