I have never been able to get a great deal with Priceline.
I just went and tried for our November trip, and the best price they came up with was $354 and I already have a reservation with the SAME company for $283.
There are a TON of coupons available on the Internet....one place that is Disney related and has quite a few is
http://www.mousesavers.com/transportation.html
If you don't find one that is a good price, type in the name of your preferred rental company and the word coupon and see what pops up.
I got my deal through my bank (USAA) also check your credit card company's as often they have coupon codes too. Even our insurance company sends rental ads with our monthly statement, but the bank codes have always been better for us.
One last thing to consider....since you're flying into San Francisco I'm going to guess that you plan to spend time in the City before driving to Southern CA. It is ALWAYS cheaper to rent a car off-airport, so if you're staying with family consider asking them to pick you up and take you to an off site rental. Sometimes it's even cheaper to take BART from the airport to whereever you're visiting and rent then....do the opposite when you return, drop your car and take BART to the airport. BART is a fun ride for kids that have never been on a subway type train.
Bring along lots of things to keep the kids entertained in the car....assuming that you're driving down Highway 5 since it's the quickest route...there isn't much to look at along the way. The coastal route is a prettier drive, but a couple hours longer.