I buy them what I think they would like, I don’t bother too much about spending the same amount on all of them, within reason.
Oldest son is getting a Nintendo switch with the bits and bobs and games to go with it.
Next eldest son is getting an apple laptop, not sure which one but they’re expensive, I’ll get him one though and it’ll be looked after.
Next son just wants a quad bike which he will get but I’m leaving it to dh to sort that as I haven’t got a clue!
Dd1 wants a “real pony and a husky” neither of which she will get 😂 but she also wants art stuff and clothes and books and maybe a speaker for her room.
Dd2 is getting a bike and lots of dresses!
Ds4 wants a pug! I’m considering it 
Dd3 wants the same sort of thing as dd1, she likes bits and bobs, clothes, art stuff, she’d like a speaker too , she’s only 4 but she’s started to love her music 
So as you can see we have a lot to buy for, and we don’t hold back but we can afford to be this way, of money was tighter then I wouldn’t feel bad about setting a budget for each of them.