Yes, We rented a car with the same group count that you have and it really worked out. Sometimes people wanted to get away and others wanted to stay. The freedom of a vehicle and the low cost was worth it. Likewise, the driving was easy in the area and we did a lot of sightseeing we would not have been able to do without a car.
Last time I was there, Coronado Marriott charged $28 for valet parking but self-parking is around $22 with in/out privileges. There are parking garages operated by ACE that are comparably priced, but I would NOT recommend that with a rental unless you are absolutely certain 3rd party liability is covered by your auto insurance carrier or the policy on your rental agreement.
For example, I have a business associate from Pittsburgh, Pa that came to NY for a wedding, rented a car at LaGuardia and used an ICON parking garage in Midtown. While one attendant parked his vehicle, another attendant was retrieving a car and a fender bender happened. He declined coverage favoring the umbrella policy from his personal auto coverage. Turns out that his personal policy doesn't cover damages incurred when you turn the car over to a valet driver or parking attendant. Although ICON was liable for both cars, he still had to pay the rental company for repairs until he was reimbursed by ICON's carrier. That took over a year.
You don't even want to think about **** like that on Coronado while with your family for a few bucks, do you? For peace of mind, pay the extra $ for the Marriott parking.