Appointee

If a 16 year old needs an appointee to manage there finances are there rules as to what the money can be spent on. For instance, if a parent is the child's appointee can the parent spend the money how they please or do they need to keep receipts and prove it was spent on the person themselves. If they went out for a meal and the bill was £50 that obviously wasn't just for the 1 person . If they did the weekly shop and took the money from the person they are appointee for then that is also not solely for them. Can they do this? It's a bit hard to explain, I hope you understand