I have a spreadsheet, going back years and years now and have always tried to keep to the same budget.
DH and I were both brought up with lots of presents under the tree and a stocking so we do the same. I don't like the idea of 'something you want, need, wear, read' thing. It seems very limiting. And we don't treat our DC randomly through the rest of the year.
So I aim for between £150-£200 inc stocking etc. I try to get it as low as possibly under that and don't view it as a target.
My DC are teens now. They are modest in their wants though. We aren't an iphone family, so they wouldn't ask for something like that and they know it's too expensive. They know they will be able to buy themselves that kind of stuff when they are older or save for it. DD2 recently bought herself a PS4 for example.
This year, they will both have little bits and pieces for them and their rooms etc. And mostly would like cash! So the amount of bits will reduce what cash they can have.
DD2 is having a PS game, a throw for her bed, a hoodie and some other bits and pieces.