We took my daughter last year right after she turned two...and I have to say it was VERY difficult! She is a pretty laid-back, sweet kid, but had lots of issues in the parks. I don't think there is any way to know for sure how they will react to such a stimulating environment (even other theme parks, since we had taken her to Busch Gardens before with no issues!). But, here are some things I learned:
1. I didn't think about how much my daughter would be in the stroller...and she hated that! I wish I had gotten her used to spending more time in the stroller leading up to the trip. We ended up holding her A LOT - so much, in fact, that I ended up hurting my back on the trip
2. Going back to the hotel for afternoon naps was a total bust for us. We'd rush to get out of the park right after lunch time and she didn't nap most of the time and then getting back out to the parks was such a pain. We wasted so much time! But this might not be as much of an issue for you, being on the monorail.
3. She wasn't really scared on any rides, even Haunted Mansion. I don't think she "got it" at that age. But we're taking her again in November and I definitely think she would be scared now! So, that can be a plus with younger kids!
4. Her night sleep was very poor during the trip (she had been a great sleeper prior). I rented a crib, even brought her crib bedding from home, but she would cry at night and eventually ended up in my bed for the rest of the trip. I didn't get a lot of sleep!
5. Child swap is great...when it works! We had one issue where a CM wouldn't give us one for BTMR because we got there as soon as it opened, so by the time the first group was finished, the next group had to wait in a much longer line. But, other than that, and sometimes taking a while, child swap is a nice option with little ones!
6. It's great that they eat free from your plate - and it's so easy to feed them at buffets, if you snag a reservation! But, they might not like the characters - we went to Garden Grill for breakfast and she was a bit scared of the characters, but certainly not crying or anything. I think the characters are really good about respecting and understanding each kid's boundaries and they kept their distance once they realized she was scared.
7. Find free-range playgrounds to let them run around! The indoor playground at Dumbo, the small playground by Splash Mountain, etc.
8. My daughter's favorite ride, by far, was Turtle Talk with Crush, lol. She was way too young to understand any of the jokes, but she loved hearing everyone laugh and would laugh right along with us! You never know what they will love!
9. I said I wouldn't take her again until she was 5 or older...and yet, we're taking her again in November! But I think we have a much better handle on how to plan a visit with her, her likes/dislikes, and how she will react to that kind of environment. So, I'm hopeful it will be a more enjoyable trip! I don't mean to point out so many negatives from our trip, but I do wish I had considered these sorts of issues and adjusted my expectations accordingly!
I hope you have a wonderful time!