On our trip last September our DS was 4, and DD was about to turn 6 in 3 more weeks. DS is very sensitive to loud noises and "scary" sights ... but likes fast moving things. DD is pretty much up for anything except drops that make her tummy hurt and scary sights. We took them on the train that goes all around the park to let them see how big
Disneyland is prior to going on any other rides. That gave them a good opportunity to talk about anything they wanted to do right away. Another thing that we did while in line was tell the kids that if they wouldn't really know if they liked the ride until they tried it, and if they didn't like it we wouldn't ride it again ... but if they DID like it, we would go on it as much as we could

We avoided the words "scary, spinning, roller coaster, dark" and used "fun, goes as fast as xxx" and kept everything very positive.
Honestly, if we had to do it again I would have waited to take them through the Fantasyland rides until we had pre-screened them ourselves. For the most part, the indoor rides scared the boogers out of BOTH kids (very dark themed, especially Mr. Toad and Snow White). Peter Pan was a favorite (five times!!) and so was the Casey Jr. Circus Train (3 times!!). The only other rides they enjoyed in Fantasyland were the carousel and the teacups.
They both really enjoyed the Buzz Lightyear ride, Jungle Cruise, It's A Small World and the train that goes around the entire park. They did not care for the Roger Rabbit car ride in ToonTown (it didn't scare them, but was kind of lurchy/jerky), but DD absolutely LOVED the coaster in Toontown. We did Pirates, which they *expected* would be scary ... and they actually wanted to do it again! No telling what they will enjoy.
All of that said ... when push came to shove, they both absolutely LOVED California Adventure! I am glad that we did Disneyland first, but on this next trip we expect to spend more time in CA than at DL. If your kids are used to carnival rides, there is a section of CA that has carnival type rides on a larger scale (do head for the line on the RIGHT as opposed to the line on the LEFT for the ferris wheel - they will probably like the fixed cars better than the sliding ones). These rides are a lot of fun, not enclosed in the dark, and will likely get them in the right frame of mind to try new and different things at DL.
If they liked the Monster's Inc movie, be sure to do the ride at CA. We did that one FOUR TIMES, LOL!
My kids liked CA better than Disneyland, if you can believe that. The shows, the water play areas, the "kiddie" section with whimsical themed rides (like the "smell-o-ride" caterpillar). We are heading straight there in September!
GL with your trip!