Yes, you can usually get away with unloading the bags and family at the curbside checkin and checking yourself in as well (bring a tip for the skycap) while you idle the rent-a-car at the curb, then return the rent-a-car. But if there is a long line (e.g. Terminal 1/Southwest) you won't be able to do this. In such a case go return the car without checking yourself in, have the family check in at curbside (assuming there are <= 2 bags per family member) and return the car and then check yourself in. In either case have them meet you at the departure gate.