Welcome to the website.
Having worked at DL, I can tell you that the week between Xmas and New Years is the busiest week of the year.
If you are going to come, try and get to the parks at opening. Maybe do that 2-3 days at DL and 1-2 days at DCA. At DL, I really wouldn't suggest you stay more than 4-5 hours during the day. It will just get too crowded. I would breakdown the days like this.
Day 1: DL: do Fantasyland/Toontown first thing. FYI Peter Pan and Small World have FastPass. That may take you all morning to do just Fantasyland/Toontown. Have lunch and stroll around. Leave. If you have extra energy, hit DCA or DTD. Day 2: DL: Do the Westside of the park, Adventureland/New Orleans Square/Critter Country. FYI: Indiana Jones, Pirates, Haunted Mansion, Splash and Pooh are all FastPass Attractions. I would head all the way to Winnie the Pooh the work my way back. Depending what your 5 yr old can/will ride, this may take you 4-5 hours. Have lunch;explore for a bit and leave. Day 3: DL: Whatever you missed. Maybe let your 5 yr old decide. Day 4: DCA. Flik's Fun Fair/Bug's Land is great for kid's and so is the Redwood Challenge Area/Brother Bear Storytelling. Soarin is a must and Aladdin the show. DCA will get busy, but the park has wide walkways and some nice hidden areas for rest and relaxation.
Come back at night and see the Electrical Parade one night at DCA and the Parade and fireworks at DL one night. Again both parks will be busy with DL being extremely busy. DTD is in between both parks and a nice place to walk around and grab lunch or dinner.
Good luck and have a good time.