I have a 10 year old and a 12 year old.
The 10 year old wears jeans, leggings, shorts over tights etc. paired with t-shirts, hoodies (she favours zippy ones rather than over the head ones) and cardigans. Favourite shops for her are Joules, Mini Boden/some Johnnie B, M&S, Next, Zara, H&M. We've had some cracking TK Maxx finds too - including a £25 Ralph Lauren blazer!
The 12 year old wears jeans, leggings, shorts (denim only for her), short skirts (usually skater style, she has a couple of more fitted ones - generally in jersey type fabric), dresses over leggings (again jersey style, skater dresses). She pairs with vests and cardigans, t-shirts and hoodies etc. She favours Johnnie B, Jack Wills, Hollister, Zara, some H&M. She has some bits from River Island kids section, and some small topshop and New Look.
Shoe wise they both have canvas trainers (converse/vans), Uggs, ballet flats in various colours for smarter occasions and for when the weather improves and shoes/trainers for school. DD3 (12) has some leather ankle boots which she loves.
I love H&M, Zara and M&S for them. DD3 (12) won't wear M&S - she's too influenced by school mates and her older sister, despite the fact the clothes follow a very JW, Zara type style. TK Maxx is always worth a try too!