Hi!
May is a wonderful time to visit Walt Disney World! I'm sure you're going to have a wonderful time there!
From my experience, weather in late can be quite warm during the day, averaging in the mid-80's (low 30's Celsius). Nighttime temps cool off a bit and are usually in the mid-60's (18 Celsius).
This means the days are perfect for the water parks, and both Blizzard Beach and Typhoon Lagoon are scheduled to be open during your stay.
Crowds are typically lower than the summer months of June, July & August when most U.S. schools are on vacation.
During the weekends you are visiting, Hollywood Studios will be hosting
Star Wars Weekends. This is a super-fun event for any Jedi's in your family, but it also means that you may want to avoid this park on Friday to Sunday if you don't want to take part in any events!
Hope this helps!