They want to go to all the shops. Are they in walking distance or do they have a bus the girls can take? Also, is the grocery store close enough to walk too? Or will they deliver groceries like Garden Grocer? What about buses/shuttles from the airport to the resort?
I agree with PP - Aulani without a car is tough, and I would categorize Aulani as a family resort. A bunch of 18 year olds with no car may be bored, especially at night.
There are maybe 2-3 shops across the street from Aulani, which includes a small grocery store and a few others which are nothing to write home about. Ko Olina is a planned luxury vacation development, so you won't find malls or anything like that without going on the freeway. In Kapolei, there is a mall and some outlets a little further away which they could probably get to by taxi or Uber for maybe $20-$30? Just a guess though. As PP mentioned, the main shopping area is Waikiki which is a good 30-40 minute drive away. Not sure of the buses but considering the Ko Olina location, not sure if that would be viable.
There is one grocery delivery service but I don't know much about it - perhaps someone else can confirm. The ABC Store across the street does not have a huge selection but if they're just in need of snacks and drinks, they would be able to make do. Or if they take a taxi to Kapolei, they could probably stop at Safeway or Target too.
There are shuttles that can be arranged from the airport but not certain of cost a we have always rented a car. A taxi would be another option.
If I were 18, I would want to stay in Waikiki. Shopping, beach, action. They could book tours from their Waikiki hotel which would have shuttle pick up and drop off. We did two organized tours from our Waikiki hotel - Diamond Head which they may enjoy and Pearl Harbor/Swap Meet. Pearl Harbor may be iffy for teens but they might like the Swap Meet which has some cheap souvenirs and snacks. There are also restaurants for every budget in walking distance - cheap quick eats like Wolfgang Puck Express, chain restaurants, and just casual sit downs too.