The works ship is great for cheap crafty bits as is wilkinsons.
Get an old shoe box, cover it in sparkly paper, then scrunch up some tissue and lay in some small cheap craft bits, you could fill a box for £5/each child and the presentation will look really special.
The works also do small blank canvases if you have budding artists which I think is a lovely present.
Have you been on the bargain threads, there are loads of things on amazon with free shipping.
The book people alwYs have great deals, like a pack of 10 books for £10 quid which I buy then split up.
Argo has 25%off meccano at the moment for your ds! my ds would like that.
I would do-
Ds-
hex bug £9
Meccano £10
New jumper- £8
New pjs-£8
Crafty bits £5
Chocolate £5
Canvases £2
Books £5
Joke stuff from a joke shop, like a whoopee cushion, or a ring that squirts water, or weirdy glasses £2
Dd
Play doh £2
Crafty bits £5
Doll £9
Doll clothes £5
Drs set £5
Chocolate. £2
Pjs £5
New dress £5
Book £5
Hair clips etc £2
I think that would be heaps and a really nice bundle and of you look on fb selling sites you will get lots for very little. Tesco, primark are cheap and have nice clothing, accessories.