It really depends on what they like. But I would definitely say avoid any of the scarier rides (Snow White with the witch everywhere, Pirates of the Caribbean with the loud noises, etc.) at the very beginning. Not that it was incredibly scary or anything, but my very first trip to WDW was in 1993 when I was 9, and I hated being in the dark or with weird lights flashing on me. First ride we went on was Delta Dream Flight, and there was that weird segment with the black and white swirling lights that, at the time, seemed like it would never end. Freaked me out/ruined the day/made me very untrusting of Disney, and it wasn't until the next trip (in 1996) before I would ride anything other than outdoor rides. Whew, bad memories indeed.
My suggestions for you (in no particular order) would be:
It's A Small World
Peter Pan's Flight
Many Adventures of Winnie the Pooh
Dumbo (I'd try to do this early regardless-- assuming they want to ride-- since the line can get so long)
Mickey's PhilharMagic (that'd prob. be my choice if I had a 5 year old-- or any child for that matter, LOL-- assuming they like Lion King, Aladdin, Little Mermaid, etc. and wouldn't be scared at things "popping out" at them; otherwise, I don't think it would be scary to a 5 year old; obviously not a ride, but still completely magical, even to this 22 year old, LOL

)
If you'd prefer for them to work up to things, the carousel or tea cups might be a good choice; or the WDW Railroad. Or if you think they wouldn't be intimidated by the characters and would enjoy meeting them, head to Toontown Fair and try to meet some.
For me, I'd prob. go with PhilharMagic, but if I
had to choose a ride, I'd say Small World. Good luck, and have fun!
