Thanks for the feedback folks! I'll take a closer look at Regent - that's good to know there are likely to be other young people on board.
Since I first posted I've been to see a cruise agent, and she recommended the yacht club on MSC. She said that way we can get a quieter environment with some small ship advantages, but there are still all the big ship facilities available, and plenty of young people on board for the girls to socialise with. I've had a look and it seems like quite a good idea, and the yacht club prices are quite competitive compared with luxury cruises.
DD1 was very keen on the more European flavour - she's a polyglot so the idea of a choice of languages of newspapers, the library will have books in a variety of languages, she'll meet people from all over to talk to, that was very appealing to her.
It also looks like a good idea because being a big line there are lots of departure dates available. I'm a teacher so we have to go in school holidays, so having lots of departure date choices is helpful. And on some of the itineraries you can choose where to start/finish, so that will make it convenient choosing a place we'd like to take a pre-cruise stay. I'm keen to visit Barcelona, and they had several itineraries that could be taken from Barcelona, with a good choice of departure dates.
I think I'll pop over to the MSC threads and have a good read!