Our two youngest have been going to WDW since they were each 6mos old. We have never had a problem. They are 17mos apart in age, so they pretty much like the same things.
We just always take things a little slower now. We do not do commando Disney anymore. They are now 4 and 5 1/2. If the kids want to sleep a little later, that's OK. When they were younger we were always sure to bring a favorite toy or blanket in the stroller. This helped between attractions. Epcot was always a favorite for us because of the toddler water areas on the bridge going to the World Showcase. These were little shooting fountains where toddlers could play and cool off. Burn off a little excess energy. I always tried to keep a change of clothes and plenty of pullups in the stroller. During the hottest part of the day, we always tried to do indoor things--the Land pavillion, the Living Seas, places that had several things under one air conditioned roof. There are also fountains like above at Downtown Disney. My DH would watch the kids play while I got to do a little shopping. Then after wearing them out, we would quickly eat lunch and they would nap in our double stroller while DH, DD/12 and I could finish enjoying Downtown Disney.
At MGM they always liked the Little Mermaid show, a trip to Honey I Shrunk the Kids Playground and Muppet 3D show. Throw in a character sighting here and there, maybe a character meal or two.
MK is fun, but remember that most lines are outside, so it can be a little tough waiting in long, hot lines. But the reward is watching your child's face as he/she sees all the magic for the first time. We always tried to do MK in the late afternoon. Our DS has always been early to bed, so he had never seen any of the nightly electrical parades at MK until last month.
It was just unbelievable watching his face. I watched him more than I did the parade!!
With an older child, we had to find a happy medium. We did the bigger stuff with her, so she had to tolerate the little things for her brother and sister.
So far, all has worked out great. They are getting bigger now and we are moving on to bigger and better things. DS rode the Tower of Terror and the RockNRoller Coaster last month while at WDW. DD/4 was upset because she wasn't tall enough for the coaster. She isn't scared on anything.
If you take your cue from your child, rest when tired, don't try to do too much in one day, let them have a little freedom(such as playing in the water) things should work out great.
Just remember, it will never be the same as when it was just yourself and your husband.
It is so much better now watching through the eyes of your child!!!!!!!!!!!!!!!!!!!!