If you plan to visit the summit, there are a couple of options - driving to the top yourself, driving to the visitor center and then hiking to the top or taking a guided tour from either Kona or Hilo. We chose to drive ourselves (this definitely requires 4-wheel drive - heed the advice!), which requires a stop at the visitor center (preceded by a stop at the Haleakala National Park playground) to acclimatize for at least half and hour before continuing to 13,800. Being from Florida where anything above sea level seems like a mountain, Ellison and I both definitely noticed the difference in altitude. It was recommended that only children older than 16 progress past the visitor center due to increased chances of altitude sickness, but we decided to risk it since we knew we weren't staying more than an hour (there is 40% less oxygen than at sea level at this high of an elevation).
The remainder of the drive up from the visitor center consists of a winding, mostly rocky road. Since we caught were here on a particularly clear day, the views extended all the way down to the vast expanse of Pacific Ocean - it was incredible.
Ellison and I really wanted to go on a stargazing tour, but younger children aren't allowed because of the previously mentioned altitude issue. Too bad. It is definitely something we put on the list for future visits, though. I guess way in the future since you have to be at least 16.
We intended to hike around a little more than we actually did, but it was very windy and cold. Add to that Ellison's altitude headache (which I think was more likely due to the cold wind, but whatever, it was still killing her and she wanted back to the car asap) and our time on the mountain was cut short. But still long enough to take at least one family selfie to document our time at the top of Hawaii.