You will be almost walking distance from Kahaluu Beach Park. I'm not a huge fan of snorkeling but really enjoyed it there. It can be crowded but it's safe and you are definitely going to see a lot without going out very far. I have underwater pictures of my husband literally swimming beside a sea turtle. You had to be careful not to touch them one year when we were there.
You are also closer to the beach by the City of Refuge, which is good for snorkelling.
Hapuna Beach, to me, is not a place I think of going for snorkeling. It's just a great time to sit on the sand, love to body surf or boogie board there. Pack a lunch, a beach chair and a book and it's a day worth doing.
And Mauna Kea is fabulous, but the parking is super limited. Unless it's been expanded, the hotel controls the parking and they have about 20 to 30 spots a day that are available to the public. So if you don't get there really early, you're out of luck. Plan to go there early on a weekday.
As for where the teens will be, neither Paniolo or Kona Coast are "destination" type resorts. SO I can't really suggest that one would be better than the other for your kids. We did find that the pool and hot tub at Paniolo were really small for the size of the resort. It was almost impossible to get into either.