Rainbow Fairies aren't picture books. They're very, very short easy reader chapter books with line art. (They're also incredibly inane.)

There are books like this for older kids (the term is hi-lo, I think) but Rainbow Fairies is probably in the "so young it looks insulting" category. If we had any Secrets of Droon I'd probably send it along (it's slightly edgier fantasy, same basic reading level.)