I have done this for my neice in the past and do a top up every year.
Don't forget basics like glue, spreaders, paint brushes and sellotape. I bought a plastic 4 drawer tower from argos and themed each drawer. I was in tesco direct and they had tubs of sparkly pom poms, feathers or ric rac for £1. Paper Mill is good for paper and card as you can get a pinch of all sorts of colours, textures and weights. Do you have an independant haberdashers?? Ours is mainly cheaper than hobbycraft and sometimes the range.
Decorative edge scissors are always popular, i bought a set of eBay that were hardly used £3 ish inc p&p.
Ribbon can be expensive, our cake decorators shop is the cheapest and she is happy to sell short lengths as it's all on rolls.
Buttons, charity shops are often good for these.
Glitter is always popular, but find that glitter glue is less messy.
Btw neice has asked for a sewing themed edition for christmas this year been raiding our haberdashers oddments and offcuts bin, found some good bright and cheerful cotton prints. Her mum is far from crafty and struggles, so it's nice to let her have some bits and bobs.