I just pre-ordered my copy of the UK version (children's cover) this morning, and can't wait to receive it in July!! Then I'll get my American version when I go visit family for Christmas.
The UK version is different to the US version, I have both sets and the UK books use British slang, whereas the US do not. I think it's just an aesthetics issue that they produce both an "adult" and "children's" cover in the UK, the books don't actually differ (from what I've been told).