In the past I have hired a Dodge Caravan or Nissan Quest from the airport I would recomend booking it before you get there from these sort of places.
www.discounthawaiicarrental.com/ www.carrentalhawaii.com/These style of mini vans have plenty of options with folding seats for boards and people to sit.
Personaly I wouldnt go for the cheaper windsurfing van rentals that some might suggest, from what Ive seen they can be pretty average condition for a family trip.
As far as gear rental you have plenty options depending on your prefered brand.
Second Wind , Matt Pritchard, Adventure Sports, Hitech Maui, Kanaha Kai, and more.
They all let you swap gear as needed so you shoud have know trouble getting sorted, just be prepared to be on a 4m a lot!
Have fun mate!