if kids are worried about rides,i would be cautious of is telling them that, "it'll be fine", or "you'll have a great time, i promise", or "there is nothing scary about the ride", etc...
my 3 went through random phases of being scared of certain things. 3 things that seemed to help the anxiety:
1) ear muffs to lessen the constant, incredibly loud noise on some rides (ear plugs would work for a 5 y.o., i would think)
2) small glow stick to hold and look at if they start to feel uncomfortable in the dark
3) and this one was key, i think...tell them what the ride entails...there are SO many attractions that are loud, have things jump out at you, have water splashing at you, have the seats move unexpectedly. i can understand why little ones are apprehensive about what is to come:
Pooh ride: tigger jumping up and dropping down, the honey pot starting to bounce, the sudden thunder and lightening in the storm scene
Buzz: lights, loud, the seats spinnning unexpectedly (my kids hated that part near the end where the seats spin around and we were in that long, blackened tunnel with the crazy bright light at the end!)
THe Little Mermaid show: water splashing on your face
J. into Imagination ride: VERY loud , sudden sound (and bright light?)
etc, etc, etc
Teacups, Carousel, and Small World are great starter rides because they aren't crazy loud and there is NOTHING surprising.
If kids are apprehensive and you don't remember the details of the ride, you can always ride it first, then you can promise them that you'll tell them what is coming next (we always whisper our way through Maelstrom and pirates with 2 of our 3, just so they know what is coming!)
Hope you have a great time,