I've seen all three. They are all wonderful. However, if you are only going to see one, and with the ages of your boys, I think KA is the best bet. Here's why:
1. It it technologically amazing. The stage, the set, the costumes--it is jaw dropping. I think it's the most impressive to boys.
2. There is an actual story that everyone can follow. The other two have a "story" but it's really hard to understand, and frankly, I'm a smart person, and I gave up about 1/3 of the way in. KA is about a brother and sister who are seperated and have to find each other--all while going through some very scary things

3. The music is the best of the 3
4. I cannot stess the just utter amazingness of the production. There are scenes that make you tear up becuase they are beautiful and unbelievable--just wonderful.
Now, one argument that people have about KA is that there aren't as many human feats of coolness--this is true. They aren't as many contortionists (sp?) or crazy people flipping around etc. It relies on the humans interacting with the set to make the impact. Also, there are slow parts in all 3--it isn't exciting the whole time. KA is the most expensive of the three as well. I would suggest Mystere after KA, because of the boys, but O is pretty darn cool as well.
I hate to give anything away, but I could PM you to give examples
