I've made the walk, too, with a 3-year-old in a stroller, and it took 10 minutes, 15 tops. Since you'll be going in December, the weather should be nice making it for a more pleasant experience (I did it in summer heat). I will say this: walking from the Contemporary to the MK is far, far better than taking the monorail. The MK is the LAST stop when taking the monorail from the Contemporary, stopping at the TTC, Poly and GF before getting to the MK. During the holiday season, I could see it easily taking you a half hour to get to the MK via monorail. The walk will be much faster, and significantly less stressful (unless, of course, you're a monorail junkie and love the ride no matter how long or crowded, which some people do).
On the return trip from the MK, the resort monorail will stop at the Contemporary FIRST, so if your feet are tired, take the monorail back (unless you're leaving right after the fireworks or parade and the lines for the monorail get backed up).