Can't speak for the 9 year old, but some ideas that my almost 7 and almost 4 year old DDs love are...
7 year old
Books - You can get some great box sets of books for about £5. I've ordered the magic fairies and High school musical ones for my 7 year old. She also loves Where's wally too.
Clothes, jewellery, hair and make up bits. It seems big hair bands are right in fashion now, and my DDs love them. Also little packs with lip balms and nail polishes in etc.
DVDs. Slightly over budget at £6 each, but amazon have been doing two Disney DVDs for £12. I ordered Camp Rock 2 and Princess and the Frog. Obviously will also depend on TV region too.
Crafty stuff. My 7 year old has just started getting into making things, sewing bits and the like. She would love anything like that.
All things Hannah Montana. She is very in at the moment. I've bought a triple pack of Hannah Montana CDs on Amazon for £5. Quite impressive for £5.
Stationery. She loves stationery and is just starting to keep a diary - so something like that would go down well.
4 year old.
My 4 year old loves all things like dressing up clothes. Doesn't have to be the outfit, as she also loves the shoes, the tiaras and the like. These things are really quite cheap in Tescos.
What about board games - 4 year olds are often just getting in to them, and you can buy home versions of famous brand games for about £5 at Toys R US.
Jigsaw puzzles are also a fave at that age.
Also, my 4 year old DD loves Polly Pocket and all things Barbie. There is a toy sale on at Tescos at the moment, and Toys R us have had a lot of High School Musical stuff reduced so you can pick up a HSM doll for about £5.
Good luck!