I don't think you can do too many. I took Ashley at age 3 for 3 weeks but we broke it up. I wouldn't do more than 2 full, long days in a row without a break. You can do a waterpark which is a totally different, relaxing experience then go back another day. I would plan app 5 days in the parks (or 6). It is nice to go to each park once then on one of your last days go back to your fav park and redo some of your fav things and see what you missed. It's a push to do Magic Kingdom or Epcot in a day but Hollywood Studios and Animal Kingdom are doable. Epcot is app 4 times the size of Magic Kingdom.
Do you plan to go to the other Orlando parks? Sea World or Universal? For a first timer 5 year old - I would probably skip Universal but Sea World is nice. But just the parks at Disney will give you PLENTY to do. If you go for a week - you'll find its just not enough.
Each of the parks has areas and things suitable to kids. Epcot has the least but the kids still love it. Especially the Finding Nemo - Living Seas Pavillion.
