I have no idea if there were also legal issues involved, but as a big fan of the original books I can tell you that the characters look almost 100% book accurate and Dorothy was an 8-year old girl in the books. The movie itself is based on the 2nd and 3rd books in the series.

Also, if you're into whimsical children's novels from the early 1900s, I highly recommend reading the original book series. The original MGM movie butchered them pretty badly. There are 14 books written by Frank Baum, they're a lot of fun!