Hello Kim! Welcome to the Disney Parks Moms Panel.
The trials of being taller than average are well known to me Kim, for I am the same height as your niece and nephew. The good news is that there are no maximum height limitations on any of the attractions at the
4 Walt Disney World Resort Theme Parks. As a frequent visitor and kid-at-heart, I have experienced every attraction that the parks currently have to offer and while it hasn't always been pretty, I have never been unable to enjoy an attraction.
A few rides are not always comfortable, however. Despite the Astro Orbiter promising a ride in a gleaming 2-passenger spaceship, when 1 of the passengers is 6'5", they should be riding on their own or with a small child. I still recommend the Astro Orbiter, but speak from experience when I say: 1 tall adult per spaceship!
Also, despite Space Mountain being a roller-coaster-type ride in the dark, I find that only the first seat on each six-person rocket adequately accommodates my legs.
If you plan to visit the wetter side of Walt Disney World Resort, The Bay Slide at Disney's Typhoon Lagoon has a height restriction (60" or under). In lieu of this, I would recommend learning how to Hang Ten by signing up for one of the
Surfing Programs at Typhoon Lagoon. There is no height restriction when you're catching tasty waves (as Crush would say) in North America's largest wave pool.
Any other
restrictions on attractions are minimum height requirements and should have little impact on your niece and nephew's experience at the Walt Disney World Resort.
Have a Magical Vacation!