You can wave at the farm on your way to Two Step.
Everyone is going to want to see the volcano. But they have closed off the best viewing location. And even if you wake up at oh-dark-thirty, all you're going to see is the glow from the lava in the crater. I've walked several miles in the dark recently and could find no place to see actual lava pools in the crater. My friends were unbelievably disappointed. "We woke up in the middle of the night and drove for hours -- for this?"
Next time I go there I'm bringing "adhesive on a stick" and cleaning up all the money that idiot tourists toss into the steam vents, right next to the sign which reads, "please don't pollute these steam vents by tossing money into them." A few months ago, I counted about $100 in small bills.
If your boys are fit, they will absolutely love the Captain Cook trail to Kealakekua Bay and the Captain Cook Monument. It's 0.8 miles. But it has significant elevation change. I've seen plenty of seniors hike the trail (I do it every couple weeks). But they're also the seniors who look like they'd enjoy an hour on a Stairmaster.
My farm is less than a mile from the trailhead. There isn't exactly "ample parking" there. But it's also the sort of place you can drop the boys off and then everyone else go do something less strenuous -- it'll be at least a few hours.
In the area around Captain Cook, I also recommend Kona Chips (best potato chips I've ever tasted), the Painted Church, the Greenwell Farms tour (a nicer coffee farm than mine), Kona Joe's (also a nicer coffee farm than mine), Puuhonua O Honaunau (city of refuge, next to Two Step).