My 12 yr old is crowd averse and anxious, thinking this might be a god option to see fireworks/shows without worrying about being crowded. Is it worth it?
IMO, in general, no.
If the parks are too crowded to feel comfortable, it is possible to watch the fireworks outside the park. Now it isn't quite the same views, but IMO, the worst part of the fireworks crowd is often what happens when fireworks are over. AFTER the fireworks, there is the mob of people moving about the hub then exiting the park. Best to avoid the whole mess.
If you want to be out of crowds, and not pay an arm and a leg, then anyone can watch them from the Contemporary. Another option is the TTC. (not perfect, but not crowded either). Simply exit the park before they even start, and you won't have to fight the crowds to exit the park. (Alternately, you can stay late in the park.)
Where are you staying? If at a deluxe, consider the Contemporary.
For a more premium view, staying at the hotel (tower, park view), might be one option, comfortably watching from your balcony.
Another option is to dine at Cali Grill, also a bit $$ these days, but IMO, that is more worth the money, assuming you can snag a table at the correct time. You want your ADR to be about 30 min before they start if possible. (I have not booked the most recent version of Cali, so they might have fixed times).
Another dinner with a nice view is Narcoosees. Some 'Ohana tables have a view, but not all do.
Ultimately though...if you pay for a party you will be stuck with paying for it, even if it is still too crowded for your 12 year old. Whereas, watching from the CR or TTC is free and painless.