Sugarcube, I can't recall what month you are traveling to Maui. We have gone several times in the last few years and like Denise M, I do often find my best rate through Discount Hawaii Car Rental, but at least as often I find the best deal through CostcoTravel (if you're a member of course). Once in a while I find the best rate directly through a car company. I actually generally find pretty good rates for our trips to Maui though when we travel in February which is our favorite time to go, we do sometimes pay more than I would prefer. But Maui is SO worth it!!!
I agree that you should snag a car rental rez asap. Book the best rate you can find for what will work for your group. With that size family (it is a family of 5 total, right??) I would book a mini-van. That's what we do when there are 4 adults in our party. If you think you will be splitting up and going separate directions often I like Denise's suggestion of looking into 2 cars.
Now after you have grabbed a car rez, here's the big tip…..KEEP CHECKING THOSE RATES. More times than not I can find a better rate later, but not always. There have been trips where the rate sky-rocketed because as Denise says, there are a finite number of cars on island and it's not like they can drive some more down from San Jose if you know what I mean.