Have almost twenty cruises on Regent and have seen many families with young adult children. Also, many Aussies and a few Kiwis.
Regent will be casual the entire trip; not shorts in the dining room but no coat or tie required. This will be a port intensive cruise so you won't be able to take full advantage of the ship. I like the ability to order anything whether in the dining room or bar. I like having not tab at end of cruise. The included excursions don't work out as a good deal if you prefer to book private excursions but if this first time to these ports having freen excursions every day is a good deal.
Have fun with whatever choice you make!