Some of the more easily accessible classical stuff - Tchaikowsky, Mussorgsky's "Night On Bare Mountain", Peter and the Wolf, Sorcerer's Apprentice. Disney movies were full of this stuff for a reason.
The main thing is VARIETY. As a kid there was always music playing in our house and I can remember listening to Motown, orchestral stuff, jazz, Beatles, film scores... One of the things I remember liking when I was very young was Billy Cobham's "Spectrum". As was said above, who knows why kids (or adults) like what they like?
Of course, they lose all capacity for enjoying muic in that way once they hit their teens and choose one tiny genre to glue themselves to for the next 10 years or so. I know I did, but that phase passed and now I'm back to just enjoying music again.