My guess is that what might lead to trouble here is not the under-18 traveller flying, or even staying (or not) in the bundled hotel, reducing the price. But showing up for check-in, with age under legal driving to get yet another bundle reduction of ticket price, by having a car reserved, for a single traveller not of driving age, would have more than one warning bell chiming.
My advice: Don't expose your kid to this risk/hassle