I've struggled with this the last couple of years. My elder DC is 16 and wants very little - a cd player, some CDs, and a few basic clothes. My younger DC is 11 and she not only wants lots of things (doesn't expect to get them all, just has lots of ideas) but is really easy to find suprise items for.
I've never minded about spending the same amount on each but I feel they need to have roughly the same amount of presents. I have a sort of formula for stocking, main present(s), hamper (food/toiletry items), sack (smaller presents). I have autism and like things to be organised and logical so it just hurts my head! Do I just do my DS's stocking half full? And just a few clothes in his sack??
What do you do in this situation?