My DS is 5, and he loves everything!!! His fav. is Buzz Lightyear Space Ranger Spin!! It's fun. He can go on everything at WDW, except for space mountain........ and he'd go if he could, but I won't let him. He did get a little scared on Big Thunder Mountain, and didn't want to do it again. He didn't ride on Splash Mountain either, it was too cold last year when we were there. I didn't want him to get wet. Other than that, I think a 5 year old can pretty much do everything. Some other advice, SKIP Stitch -- it is not worth ANY wait at all, even if it was walk on, I'd walk right on by. It was NOT enjoyable for us at all. It's really up to your kids, but my advice would be to skip the following........ at least from our experiences with our DS.
1. Stitch
2. Space Mountain
3. Splash Mountain
4. Big Thunder
5. Haunted Mansion
Some kids get scared on Pirates, but my DS loved it......... we rode that numerous times. It can get a little "spooky" I guess for just a few minutes at the very beginning......... but not bad at all, and it has a little "dip", NOT a drop, even though it makes you think it does......... it really doesn't.
If you are going to the other parks, others can help. We've not done the other parks with DS yet, we are going to all 4 this next trip in Jan. I'm sure others will give you advice, but our DS could do almost everything, and there is a lot for the little one's to do. You may want to check out some of the kids programs at the resorts. My DS went on the Pirate Cruise from the GF last year and loved it. Not sure if you have boys or girls, but there were girls on it too, and they enjoyed it. They also have another kids cruise, I think it's the albatross?? and there is a cooking class, a lot of the resorts have kids programs. If you need more specifics, let me know, I or someone else will gladly help.
Have a great trip.