We do JR shows all the time with elementary students at the theater I work at. They can handle the material just fine. They might not be able to do all the harmony, but the shows are short enough that they can memorize their lines, learn the melodies, and retain choreography. Little Mermaid Jr is solid. I also really like the updates they made to Aladdin Jr a couple years ago (they pull a lot from the Broadway version, including High Adventure, Babkak Omar Aladdin Kassim, and These Palace Walls). Honk Jr, Annie Jr, Frozen Jr, Mary Poppins Jr, James and the Giant Peach Jr, and Dear Edwina Jr are also very doable.
I would avoid Music Man Jr, Beauty and the Beast Jr, Mulan Jr, Seussical Jr, and Alice in Wonderland Jr. The first four are longer Jr shows (closer to 90 minutes than an hour). Alice in Wonderland Jr is very hard. The songs are all quite fast and stylized and the resolution to the trial is frustratingly hard.