well , tell them you want to take them all to a cinema evening out as a Christmas present and that´s it !
I wouldnt gift ipads or any sort of electronic items or expensive clothes, that´s for relatives not for friends.
I have 4 nephews and my sister understands I cant pay for all their whims .
For my friend's children I have given a good gift when they were born and that´s it.
I cant pay yearly for gifts for every single children´s friend as each have got 2 and that would be 20 gifts
Perhaps age "sweet 15" you stop doing that and take them to the cinema ?
think of an activity you like to enjoy and do together with them ?
take them to the mini-golf in Battersea park ?
Winter wonderland and one hotdog for each ?
this will make a good memory , the gift will be donated in a few years time but the experience will be memorable
regards