You siimply have to let them decide when they want to ride, but also you must know what scares them. I have pushed my son to ride a few rides that I knew he would like once he got on them. Darkness is a big issue for him to, being 6. He's getting much better lately, so hopefully he'll ride Mayan Mindbender this year.
His first big ride was Kong at USF, believe it or not. When he was 2 I believe. He wasn't scared of the dark then. He hopped on and grabbed onto the lap bar and was ready to go. He was wedge tightly between me and my wife, so he was safe. He also rode Pirates on that trip.
Dark rides have been tough. Even ones like Pooh he doesn't want on. This stems from my love of dark rides, especially the scary ones. He thinks they are all scary. He is finally getting over this, as he rode Toad and even Haunted Mansion Holiday at DL last October, and even rode the mansion a second time. It was his favorite.
Coasters are a mixed bag, he's ridden Viper at SFAW a few times. He likes it, but it always overwelms him a bit.
The biggest thing to keep in mind is how well the kid can control themselves. Like on Viper, he's tall enough, barely, but he just doesn't control his upper body much, so he gets through around in teh turns. I end up helping support his upper body. Same thing happened on La Vibora at SFOT. He got tossed aroudn at each brake, and ended up not liking it.
The most important this is to not force him on too much of a ride. ONe bad ride could cause a child to not ride other rides that they used to like.
Also, try similar spinny rides to help build up to coasters. My son has been afraid of XLR-8. We rode the Chance Aviator at Kemah a few weeks ago and he liked it. SO, I'm pretty sure he'll go on XLR-8 now.
I don't want him to ride the rides too fast. Every Spring I look forward to sharing new coasters with him.
Rastus O'Ginga