Movies in theaters right now they might like:
Ice Age Dawn of the Dinosaurs
Hannah Montana the Movie
Monsters vs Aliens
G-Force
Up
Movies you can rent:
Step Up (has some mature content but it's not too bad for their age)
13 going on 30
The Incredibles
Are we there yet? (It's a family comedy)
Are we Done yet? (The sequel to Are we there yet?)
Shrek 1, 2 and 3
What a Girl Wants (The movie with Amanda Bynes)
Camp Rock (Disney Channel Original Movie)
Max Leeble's Big Move
Akeelah and The Bee (It stars Keke Palmer from True Jackson VP)
Shark Tale
17 again
Kung Fu Panda
College Road Trip
The Lizzy McGuire Movie
Agent Cody Banks
Hairspray
Ice Age
Ice Age The Meltdown
Finding Nemo
High School Musical 1, 2 and 3
Princess Protection Program (A Disney Channel Original Movie)
Spongebob Squarepants movie
Spiderman 1, 2 and 3
Kicking and Screaming (that comedy with Will Ferreal)
X men 1, 2 and 3 (the last stand) A great movie with some violence but for their age they can handle it
Nancy Drew
Hotel for Dogs
Drake and Josh movies like Merry Christmas Drake and Josh, Drake and Josh Go Hollywood
Or if you want you can even get some old school flicks to share with them. Just get out them classic Disney Cartoons (Cinderella, Beauty and the Beast, Aladdin 1, Aladdin 2, Aladdin 3, A Goofy Movie, An Extremely Goofy Movie, Sleeping Beauty, Snow White and the Seven Dwarfs, Dumbo, Pinnochio, The Rescuers, Bambi and all those other Disney Flicks).
Other good old school movies can be:
Good Burger (that movie with Kenan and Kel was really funny I think they'll like it)
Double Dragon
Like Mike (a movie starring Lil Bow Wow)
A Kid in King Arthur's Court
A Kid in Aladdin's Palace
Honey I shrunk the kids
Honey I blew up the kid (in fact, forget this one, wasn't that good, get Honey I shrunk the kids instead, great movie, classic)
Problem Child 1 and 2
Home alone 1, 2 and 3
I hope I was able to help. Have fun with your little cousins! :)