Actually, Southwest no longer allows families to board first either. They do pre-board, but it is after those with an 'A' boarding pass. This change was made with the introduction of the new 'A Listers' frequent flyer status.
We flew with our then 2yo and 4yo to San Diego last year. We had to bring 2 car seats since we were renting a car and it was a pain. We checked the car seats in, but forgot about them at our destination until we got all the way to the rental car place

(they didn't come out on the carasel since they were considered oversized). We didn't use them on the airplane as I've seen other kids in carseats in the past and it seemed all they wanted to do was kick the seat in front of them. Without the car seat my kids couldn't reach the front seat and it left room to bring the tray table down for snacks and our DVD player...
I debated about what to do for our upcoming May DL trip (we are also flying from Sac to DL). I've decided to not bring the car seats and use DL Express as our mode of transportation.
Something else to keep in mind, car seats are also very bulky to carry around in the airport, and with your parents already having to keep track of their luggage, the kids luggage and your kids, it's one less thing to hold and keep your eye on...