I'd do 6 nights on Maui and 2 nights on Oahu. Ideally they'd stay in Oahu first and Maui last, but I see that you've booked the opposite. Oh well.
The "must-dos" on Oahu IMO are USS Arizona (aka "Pearl Harbor"), USS Missouri (I highly recommend the Explorer's tour), and Punchbowl Cemetery. They are very moving. These can all be done on one day, so 2 nights on Oahu is perfect. You want to get to USS AZ early (7am). I'd rent a car in Oahu so that they are not chained to the bus schedule.
I don't agree with folks who say that the Road to Hana is a must. While parts of it were really cool, especially the road past Hana, it's a lot of tedious, sometimes hair raising driving. If they really want to experience it, my advice is to shorten it. Start your day at Ho'okipa Beach and watch the windsurfers do their thing. Then drive a few miles towards Hana, stopping to enjoy a few waterfalls and the Arboretum. Maybe eat a picnic lunch at one of the state parks. Then turn around! No need to actually get to Hana, because there's very little there of interest. If it's a clear day, head up to the summit of Haleakala. If it's not a clear day, head to Iao Needle State park.
Options for Maui:
Old Lahaina Luau
Warren & Annabelle's (highly recommend)
Ulalena
Ziplining on Haleakala
Sunrise or sunset at Haleakala
Stargazing on Haleakala
Snorkeling trip to Molokini
Trilogy all-day trip to Lanai (highly recommend)
Snorkeling at Honolua Bay
Whale watching (highly recommend)
Helicopter ride
Kayaking
Horseback riding
Spa day