Books for early readers
I've recently found these books by Sally Rippin called the 'Billie B. Brown' series which seem to fit my criteria. They are for girls, but the girl is a tomboy who has friends who are boys. They are a nice alternative to the painful (yet, so loved) Rainbow Magic books (which, as an aside, were enormously helpful in getting my girls going with their reading).
Darcey Bussell's 'Magic Ballerina' series are a similar reading level to the Rainbow Fairy books (a step above Billie B. Brown) and another alternative to the fairy books.
For the boys, some series that I like are Louis Sacher's 'Marvin Redpost' series, 'Zac Power' and 'Flat Stanley'.
In saying that these are 'girls' and 'boys' books I don't think it matters all that much. My theory is to keep finding books that suit their reading level and just keep trying them out until they find what they like.
By the way, take an early reader to the library and let them choose whatever they like. If the book is too hard but they are fascinated by the content, let them have a go. It's amazing how motivating the desire to find out about something for themselves is. This is especially the case when they are starting to read.
If your daughter wants to read every single fairy book, even though they have the same plot line - just let them. If your son will only read books on building bridges - just keep searching until there are none left for him to read. Practicing is the key to gaining confidence with reading.
There is a lot of research that shows that a key component of high literacy levels is providing access to books. So either buy books or borrow them but keep them always available - access is crucial.
So I hope that you are all giving your kids books for Christmas (or at least visiting your local public library!). Plus (bonus parenting incentive), Reading Kids = Quiet Kids!