If your in laws plan on doing full days at the park, I suggest that you have your own car, so if your tot gets tired or overwhelmed you can leave early.
Arrive to the parks at opening time, this way you can do all the rides in Fantasyland with little to no lines.
Ride Dumbo first thing, otherwise the lines are unbearable. What we do is that I get in line for Dumbo with dd, and dh gets in line about 30 people behind us. This way, when dd and I are done, I will pass her over the gates to dh and she car ride again with him.
Don´t do Snow White unless you think your tot can handle it after all of the other rides, also leave Pooh as the last one because it can be bumpy, so ride Dumbo first, then Peter Pan (when you first get in line for Peter Pan take fast passes in case you want to ride again), and then go to It´s a Small World.
There is a small play area Pooh themed and both there and in the entrance to the Little Mermaid are water jets to play, so be sure to bring a swimsuit and a full change of clothes for your little one. We always bring and extra t-shirt or two in addition to the full change, because ice cream in Florida melts at lightning speed, so dd gets really messy many times a day.
Bring your own stroller where your child is comfortable in case he or she needs to nap. If you have a stroller plastic bring it too and try to leave your stroller covered when you are not using it.