Unfortunately, every kid seems to have different "triggers" when it comes to something being scary. Some kids don't like loud noise, some don't like the dark, some don't like the villians, one of mine was petrified at the age of 3.5 of anything in disney that used fake smoke (even the fake smoke that came out of the oven on carousel of progress almost did her in!).
My suggestion is to start out with small world and work your way up. If you start with a dark ride that gets either one of your kids even remotely scared, you may not get them on anything else afterwards!
Do either of your kids have any fears that you know of? That might give a better picture of which rides they will enjoy or not enjoy. I didn't have any problems with any of my kids at age 2, but by age 3 I began seeing more fears. My oldest was 3.5 on one trip and was afraid of EVERYTHING, my middle child was only afraid of the obvious ones at age 3 (like the HM) and my youngest is currently 3.5 and we just got back and she went on everything without a problem including Tower of Terror!