Claire's, the high-street accessories store, has up to 75% off on sale items. They also have 3 for 2 on a range of brands.
For children's hair there's also a 25 Pack Hair Elastics, super useful as my daughters seem to lose theirs all the time. These are all £1.50 down from £4.50.
Looking through what's on offer it's mostly not seasonal, so it's not like you won't be able to use it now, or would have to put it away for a different time of year. There's definitely a few great gift ideas too.
The 3 for 2 offer has a lot of One Direction, Hello Kitty and Disney goodies on offer, but three, get the cheapest free.