For sure rent a van or big car.
We’ve always stayed in both the Kona and Hilo side with 80% of our fine in the Kona side. We always get VRBO.
My favorite thing to do there is two step beach. Swim among dolphins there is great.
Tons of other good snorkeling places on Kona side. Maks beach is also cool. Captain Cook’s also a great destination.
The Hilo side has the volcano, a cool little town and the drive through the middle of the island is cool as well as the drive on the south side IMO.
Best of the Hawaiian islands in my view especially with a bunch of capable kids.