I discovered that I could understand most of this:
http://www.youtube.com/watch?v=S4U06UAYWkQ
(There's also a version with subtitles at
http://www.youtube.com/watch?v=85XtjQPhOr8 but don't cheat and watch it first ;) )
It makes sense -- since the movie is intended for children, the lyrics are kept fairly simple, nothing too difficult. So if you want to test yourself, or practice, give it a listen! If you can work up to being able to understand it without looking at the subtitles, then you've learned a lot!
Edit: The romaji is wrong in the subtitles, though.
Edit 2: And I'm not 100% in agreement with the translation back into English, though it's not awful. I'm pretty impressed by the Japanese lyrics.
Edit 3: Maybe I should try doing my own translation in a spoiler...