It really depends on your teenagers' interests. For me, I hated Waikiki as a teenager because there were too many people, too loud, too crowded, too many stores, etc. I came from a small town and the big city was too overwhelming. The only good thing was that we got to visit relatives that lived in Honolulu. I wanted to be on the North Shore, on a quiet beach, or exploring nature, not in the city.
The big question here is: Why does your child want to go to Aulani? What is it that they want to do or see there? Is it something exclusive to Aulani, or something they can do elsewhere but don't know that yet?
A one-night reservation at Aulani can get you two full days to enjoy the quieter beach (on a weekday, at least; it's busy at the weekend), character experiences, pools, etc. Will both of them enjoy that, or will you have one complaining the whole time? If the latter, perhaps going to Aulani as a day trip would be a better option: grab a character meal at Makahiki (if characters are the draw), wander around, enjoy the beach, enjoy Mickey shave ice, and then head on to something else, like a late afternoon adventure at Kualoa Ranch or whatever else you choose. It could be a nice way to reach out and see more of the island that day.