1. Why Mauna Kea? Sunset or stargazing/telescopes?
https://www.tripadvisor.com/ShowTopic-g29217-i268-k2316287-Sunset_from_mauna_kea_visitors_center-Island_of_Hawaii_Hawaii.html2. I've been to Jaggar during the day, I've been to Jaggar at night, I've been to Jaggar at dawn, and I've been to Jaggar at dusk. The changing colors, etc. is part of the magic.
5:49 dawn Monday before heli flight (even earlier one) works well. Just be there 5-10 minutes (or more) before dawn, and make sure you have warm clothing. Or better yet, get to HVNP early Tuesday and spend a few hours.
3. Akaka Falls has nice hike. Umauma, IMO, is much prettier, and/but no hike. You might have time for both.
4. Don't leave out a black sand beach, if you haven't been to one yet. Punalu'u is an easy one to access, can do Tuesday. Or Kaimu, near Kalapana, if you like more privacy.
5. South Point is utterly uninteresting, unless you like those stick lighthouses (?). I've never seen anyone jump in the winter, just in summer, but maybe I was there at the wrong time of the day/week.
6. Green sand beach best late in day, like if you could catch the last ride out. Then it's empty, no bugs, less harsh light. Prepare to eat dust. Support baby's head well on the ride out and back. Make sure baby is well secured to adult for the way down. Don't plan on carrying in arms.