A couple of weekends ago, I took my son camping for his first hiking-overnight. It was a big deal because we hiked in - it wasn't just car camping.
We invited a couple of his friends to come with us, along with my dad and step mom and one of Nick's friend's mom. Everyone carried gear - the kids even chipped in and carried their own packs filled up with their clothes.
So, the place to go is
Point Reyes - specifically
Coast Camp. There's a couple of easy routes to go: one route is up and over a small hill and it's 1.8 miles and the other round is 2.9 miles on a flat lazy path. The sites are a 2 minute walk to the beach and you can hear the waves crashing on the beach at night.
Point Reyes is about an hour drive from San Francisco.
I highly recommend you check this out, whether it's with the kids, with some friends, or for a quick escape from reality.