3 days is great for a first trip. It gives you 1 day to see each park, and 1 day for repeat rides and to catch anything you missed the first time.
If you are staying at one of the 3 deluxe hotels and have EP, you really don't need a plan as far as rides go. Head to the HP section first during early entry each morning and stay until it starts to get crowded. I'd focus on rides one morning, spell casting another, and shopping another. This way you can really experience HP without fighting the crowds. Everyone always wants to rush off to the rides first thing, but sometimes being able to move around the tiny shops freely can be a bigger advantage than a short wait.
See below for a list of things to do in the HP section:
Does somebody have a list of all the "must do" HP things in each park?
As soon as HP starts to get crowded, move onto the rest of the park. With EP there really isn't any need to plan a certain time to do anything. I'd just move around the park in a circle riding as I go to minimize backtracking.
If you are staying at a hotel that does not offer EP, I'd focus on HP one morning, and then the headliners in each park the other mornings.
As far as "must do" activities, there are a number of threads you can read through devoted to this topic, and everyone has different favorites. I'd look through the rides to see which are based on themes, characters, or movies your kids like and go from there. Also, take note of the height restrictions, as your 5 year old may not be tall enough to ride everything.
I'm not going to name any must do attractions, but I will list a couple little known often overlooked must dos for us.
Set aside some time to visit the talking fountain. This is a favorite thing of ours to do. We love to watch the fountain interact with the kids, and kids really seem to love playing with the fountain.
Soak a few unsuspecting riders on the raft ride. If you go up on top of Popeye's boat, at the back, you will find water cannons you can use for free to spray the people passing by below on the raft ride. Kids love this.