There's no way you could get away with £15 on an older child, though I think it's great to spend less whilst you can, we did too as there's no point buying for buying sake.
Dd age 7 will get £200 on her main present, £150 on smaller presents and £15 on her stocking. Dd aged 5 will get same except her main is less at £150.
This has gone up since last year as their main presents are more expensive unfortunately. I love Christmas and start planning August as I like to make a big deal of it. We are by no means well off so I budget pretty much all year round. I also only buy the kids presents at birthdays and Christmas.
In our family, we all only buy the children presents at christmas as we feel Christmas is more for them.
I think it stems from when I was younger, my mum and dad were always skint but we still woke at Xmas to a sack full and I loved it. Most of it was cheap tatt but I loved it anyway!
The important I think is to spend what you can afford and what you feel comfortable with.
However, Im not a fan of the latest craze of spending next to nothing for the sake of!- last year there were threads where people tried to outdo each other for spending less not because they were sort of cash but out of being plain tight! Personally I'd feel mean spending money all year on myself and nothing at Christmas on the kids 'on principal'!