Hi, Anna!
What a great trip to Walt Disney World you're planning with your 15-month-old princess!
There's a great resource for folks who are taking a trip to WDW with little ones on their website; just click on this article, entitled
In-Depth Planning Tips. There, you'll find information on Baby Care Centers, stroller rentals, little-Guest-friendly attractions, kids' programs and much more.
Height requirements can be found on each park's webpage; just click on any park, then "attractions" on the left side of the screen and you'll find a list of attractions at that park. I clicked on
Haunted Mansion, for example, where the Height Requirement is listed as "Any Height."
In addition to this, check out the great, free APP My Disney Experience, which alllows Guests to see updated maps of the parks, including height requirements, where applicable.
Have a wonderful trip! Be sure to send more questions as you think of them; we love to help!