We did the full tour a few years ago. We really liked it. It was interesting and informative. You go down into the dam in very crowded elevators though.
If you can only do the powerplant due to your child's age, I would. Otherwise, you are just in the visitor center looking at displays and reading. You may want to watch the first Transformers as the powerplant is in the movie.You can walk across the dam and look down
There really isn't too much else to do and you should be done in about 2 hours tops. We went first thing in the morning during the week and the crowd wasn't too bad. Also, if you are going in the summer, beware the killer heat. You don't want to be anywhere near the dam after 12. 116 degrees is not fun.
I find the story of the dam just incredibly amazing. What was accomplished 80ish years ago without computers, fancy machinery and in the middle of the desert is just unbelievable.