Hey Opie,
You have an interesting dilemma.
You might be better off with taxis. While your first day will start out at ProtecPlaya, I think the other days are going to be training in the cenotes that are just outside of Puerto Aventuras, so just a short ride. Depending on where your condo is in PA, you might have to hump your gear to where the taxis are. There's usually a few around the Marina or near the front gate. Protec will bring your tanks, so you don't have to worry about that. I'm pretty sure you can talk them into dropping you off after the course too as long as you don't need to head into Playa to do video analysis.
The cost of taxis can add up though if you're heading into Playa for dinners, and down to Tulum for the ruins (which I found very worthwhile). But not having the stress of getting lost, getting into an accident, getting ripped off at the gas station or the car breaking down or into might be worth whatever difference there might be in costs. Keep in mind, you're there for a vacation. You'll be getting enough stress on your course, so relax the rest of the time. It sounds like it's your first time down there, so this might be the best way to scope out the lay of the land. I'm pretty sure you'll enjoy the area and the diving that is offered there, so you and your family will want to return. You can rent a car on subsequent trips.
Re: renting a car (which we always do), get all of the insurance. We even pay extra to upgrade to Zero Deductible. You can look at Easyway, Cancuncarrental, or AdoCar as some of the ones we've used with varying degrees of success. Don't just rent the smallest car. You have to think about how much luggage you're going to need to transport to and from the airport. Four adults and a small child can carry a surprising amount of luggage.