This past March I took my DS (3yo) for his first trip and we had a great time. The best advice I can give, is to go at your kids pace and take plenty of breaks. When my DS seemed to be getting tired we headed back to the hotel for a rest. It worked out great, no meltdowns the whole trip! Granted we didn't even make it to Frontierland in MK but this trip was for my DS. Take time to enjoy the little things, my DS loved feeding the ducks at the resort.
Also, if you are not sure how your kids will be with the characters, go to the Character Caravan at the resort, this was a great first experience with the characters for my DS. There wasn't a lot of people around the characters and my DS could approach them slowly on his own. He loved the characters except the Queen of Hearts, he would run the other way whenever we saw her, which was quite frequently (LOL!).
I would suggest going to see Playhouse Disney at MGM, it was great. Also, try to go to a character meal.
As I said we didn't get to do some the things we have done on past trips (w/o a child) but this was by far the best trip.
Have a great time.