We have 3 young children, and want to make sure it is worth it to get the park hopper option, so we can take advantage of the extra hours. (and not have to plan to stay all day at a specific park)
Here is a list (from either this site or maybe Allears?) that has which attractions are open for evening EMH. (Morning may be different) This way, you can decide if there is going to be enough open that they can go on to make it worth it. I found this a few months ago, so a few things may have changed, but it will give you a general idea.
Magic Kingdom® Park
Buzz Lightyear's Space Ranger Spin, Cinderella's Golden Carrousel, Dumbo the Flying Elephant, "it's a small world", The Many Adventures of Winnie the Pooh, Mickey's PhilharMagic, Peter Pan's Flight, Space Mountain®, Stitch's Great Escape!, Tomorrowland® Indy Speedway Big Thunder Mountain Railroad, The Haunted Mansion, The Magic Carpets of Aladdin, Splash Mountain®
Epcot®Living with the Land, Mission: SPACE®, Soarin', Spaceship Earth, Test Track Honey, I Shrunk the Audience, Journey Into Imagination With Figment, Mexico - Gran Fiesta Tour Starring The Three Caballeros, Norway-Maelstrom, The American Adventure
Hollywood Studios
Muppet Vision 3-D, Rock 'n' Roller Coaster® Starring Aerosmith, Star Tours, The Twilight Zone Tower of Terror, The Great Movie Ride, Voyage of The Little Mermaid
Disney's Animal Kingdom® Theme Park
DINOSAUR, Expedition Everest, It's Tough to be a Bug!®, Kilimanjaro Safaris, Pangani Forest Exploration Trail® DINOSAUR, Expedition Everest, Festival of the Lion King, (1 extra show) It's Tough to be a Bug!®, Kali River Rapids, Primeval Whirl®, TriceraTop Spin
Hope this helps you decide.