as one of the astronomers who has taken this route a couple of times, let me give you a few observations.
1. you will not get in trouble just for taking a car on the Saddle Road, the road to the observatories, or even the unpaved dirt/gravel road after the visitor center and up the mountain. No one is going to stop you or arrest you. But if something goes wrong, you will probably be out a big chunk of $$ from towing or roadside assistance. On the other hand, nothing has gone wrong in the many times I've done this yet. Just be sure to drive in good conditions only, and carefully! The rental companies are simply reducing their liability by saying that you're not "allowed" to drive on these roads. Every time we go, we take cars on this road, and aren't given any special permission by the rental car companies. We don't even tell them, they just get a car back covered in red dirt... :P
2. some people even take 2 wheel drive cars all the way up the mountain. But this is not recommended. First, your car will have a slow time getting there. Second, it is bad for the dirt road. but as I say, it has been done. PLEASE PLEASE PLEASE when you drive down the mountain DOWNSHIFT into lower gear to slow down instead of braking no matter what kind of car you drive -- brakes overheat in the thinner air, and cars get into very bad accidents by going too fast downhill with poor braking! (they have a picture book at the visitor center of car accidents on the mountain if you ask them!)
3. then the more important question, do you want to go all the way up? There certainly are many cool telescopes and cool sights to see at the top, like sunset. You might be more comfortable going with a tour group, but this involves $$ and time. The visitor center halfway up the mountain is interesting (at the end of the paved road), but probably not worth the trip alone and you don't see telescopes from just there. Wear warm clothes if you go all the way up! The astronomers are always getting funny looks for bringing parkas at Hilo/Kona airport.
4. definitely, please take time to acclimate at the visitor center if you go. Altitude sickness/exhaustion affects *EVERYONE*, regardless of physical fitness or health. When we go to the mountain, we wait one night at the halfway point before going up to the top for the next night (because we are staying at the summit for several hours). At the summit, you will feel tired/winded doing just the simplest things, like walking around on flat ground! You say that you have one child in your group -- this may require special consideration. I believe there are warnings against taking kids up the mountain.
one of the tour companies has this warning: "People with respiratory, heart, and severe overweight conditions, pregnant women and children under 16 are advised not to go above the 9,200 ft. level where the visitor center is located. Do not go to the summit within 24 hours of scuba diving. "
see this page for reference:
http://www.ifa.hawaii.edu/info/vis/ visitor center
well, that's all I have to advise. hope you find it useful.
[This message has been edited by TA (edited 10-17-2003).]