The GMs party was for families. ( our daughter was 9 last year, 10 this year, so your girls would be the perfect age.) It is just a dance type party. At one point the cruise staff will ask someone to make up a dance move. Gabby did a spin, so then they asked about 5 more kids for a dance move, then they combined them all. It was really cute. My dd loved it, since they kept saying "Do the Gabby".
Sounds perfect for my girls! I'm sure they will love it. We are also traveling w/10 and 11 year old girls. They're veterans so I'm sure they already know about it, but I just told my girls what you said, and they're psyched!

