Well, first of all to answer on the 'adult' and 'children's' version thing. I have seen the adult version of the last book
and it's ditto to the children's version save the cover.
In my country we get only the UK version i.e Bloomsbury version (and it's really good

), but having said that I've also seen the US version i.e. Scholastic one (my friend has those). The words are the same in both the versions. The difference is in the presentation though. For instance, in the US version, letters which the character's write to each other are either italicised or written in a different font. The signature at the end is more real sort of, i.e the neat writing of Hermione is shown and the untidy scrawl of Ron, etc. But in the UK version it's not like that. It's just written normally. Also in the US version there are pictures at the beginnig of each chapter, so it adds to the enthusiasm of the fans.
The adult version is just there for Bloosmsbury (if I'm not wrong) and there's the difference only in the cover, perhaps because adults should feel
a little important while reading the story. (no offence to any body here)
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One big question I've always had is why they made the American versions. I've read both editions and all that appears to be different is just the simple words like Jelly instead of Jam, and Bathrobe instead of Dressing-Gown. As these were never used as major plot points does anybody know why they did this?
Ignatius- I think this is because that is the US version, hence it is written in American english, and ofcourse there are differences between the British and American english. This has nothing to do with the main plot of the story and they didn't do this on purpose.
Hope this answers some doubts.