We sailed the British Isles last May and it was a fabulous trip; all that we had hoped it would be.
My first thought was about your genealogy/ heritage. Both DH and I have our majority of heritage from the British Isles (me, mostly Scottish). So, we really enjoyed doing a bit of research and then exploring when in port. We visited some "Campbell Clan" castles and I purchased a Campbell tartan scarf to bring home.
Regardless of heritage, I think your nieces are old enough to learn about England, Ireland and Scotland. Edinburgh is enchanting. Your nieces could do some separate research and then be instrumental in booking your shore excursions. Obviously, I do not know them, but they are old enough to appreciate the ports.
BTW- We love the Regal! Have a great trip.