I would recommend "Six" the musical about the wives of Henry the Eighth.

https://thevaudevilletheatre.co.uk/tickets/six/

Quite a powerful show.

Have your granddaughters listen to the cast albums before making their choice (on Spotify or clips on-line, etc). See how the storyline, lyrics, and music resonate with them. I find I enjoy a musical much more when I am familiar with the storyline and the music. Hamilton's rap parts are quite fast so familiarity with the lyrics would be helpful for your granddaughters. I taught a 6th grade Drama class, and my students loved it when I'd show clips of Hamilton. The soaring (pun intended!) "Defying Gravity" song from Wicked will give the girls goosebumps, and they will want to sing along! Six (ages 10 and up) is a hoot with plenty of modern references! Make sure they know the story of Henry VIII and his wives...again, appreciation for the musical will be heightened.

Hope you all have a great time together!

SIX is a geat recommendation. We saw it in our local theater and the teens in the audience where hooting and hollering.

We have tickets to see MRS. DOUBTFIRE this summer; did they enjoy that movie?

If you do SIX the kids might have fun trying to figure out what pop stars each of the women are based after.
Maybe & Juliet might be an interesting play , a much different take on Romeo and Juliet..

That would be tough

& Juliet closes this weekend

Any of those would be so much fun for them, but having said that , if you are not theater goers, I think Wicked is very enjoyable for everyone, especially if they’ve seen or like The Wizard of Oz. It’s such a fun twist to the story. My daughter, at that age, was crazy about Wicked and my husband and son, who are not fans of musicals, really enjoyed it too.
If they are history buffs and you don’t mind some mildly racy references ,Hamilton is also terrific. We’ve seen it several times and each time we pick up something new. Both its lyrics and the show’s pace require attention to get the most out of it.
Mamma Mia’s storyline isn’t as engaging to me. ABBA’s songs are so very singable though if you like and know Abba. I just feel the other two will capture them a bit more. I could be wrong. It’s all a matter of taste really and very subjective. Just my opinion.

"That would be tough

& Juliet closes this weekend"
Shame ...
Guess I will just have to see it on Broadway.

Honestly I just mentioned it because a few months ago my niece took her 12 year old and saw it and the 12 year old loved it.
Honestly is it such a crime that I did not know that it was closing in a few days.