Hmmm.... Depends on what they wanna do really. Most of the large resorts in Bali have more or less the same amenities? I would strongly advise agaist Ritz Carlton Bali (in Nusa Dua), we had a pleathora of problems and issues and management was grossly underwhelming in every way. The huge chinese wedding parties and lackluster attention to details such as cleaning and upkeep killed it for us.
Both Four Seasons properties are nice, but they are OLD and need a lot of work. I know tripadvisor and other places where bloggers hang out love them both, and they are pretty, but ever ever so OLD and details are hardly like modern european, middle east or american four seasons properties. The pool area in Sayan is tiny and dingy, and Villa pools are often cold'ish due to partially being in the shade (except for a couple of the very large villas). I did not try Mandapa, but according to friends its better value than Sayan.
Our teen enjoyed Como Uma Canggu - mainly beacuse the beach is walkable, there is a few tiny backpacker hamlets nearby with a cool vibe and food/drink scene and other young people. You can actually walk outside the resort and see other things without getting into a taxi or whatnot. In general it has a more "young" vibe than the more traditional chain resorts.