I wouldn't normally use this blog to promote a particular retailer, but Waterstones have a great offer at present that seems too good to ignore. They are offering the first book for £1 in each of five different children's series with free delivery (online) or in-store. It's time limited.
For the younger ones there's Horrid Henry (of course) and Cressida Cowell's Hiccup; for readers aged 9-12 there's Skulduggery Pleasant and Artemis Fowl; and teens can get their teeth into Darren Shan's Demonata series.
Waterstones Children's Books for £1 offer