Hi Daniel!
Unlike most regional amusement parks, Walt Disney World really is designed for the entire family. Your 6 year old will be able to enjoy most of the attractions in the theme parks. If you go
here, you can get more planning information. After going to that page, look to the right for "Attraction Height Requirements". That document will show the various height requirements for the separate parks; if an attraction isn't listed, there's no height requirement!
With regards to having a good plan to handle the crowds, I suggest using a good touring plan that minimizes your waiting in lines. The best touring plans, in my opinion, are produced by the team behind "The Unofficial Guide to Walt Disney World". Their website is TouringPlans.com and their recommendations are based on a lot of math and years of data collection. As an engineer, I'm a big fan of their work and I've personally tested several of their plans. I suggest giving them a try!
Enjoy planning and write back if you have any more questions!