I have kids and was asked once if their were firearms in my house prior to a play date. I told the other parent the truth and then explained that they were all locked in a large safe. The other parent didn't have a problem once they knew they were completely secure. I don't think I'd tell anyone unless they asked. You know everything is secure, so keep things to yourself unless specifically asked. Why raise unnecessary concerns?
As a gun owner you are legally liable if a minor gains access to your guns. Make sure they are secured and that can't happen. Beyond that if the other parents ask, then tell them the truth, including how you have them secured, etc.
I also live in CA, when my kids were young I had exactly one mother ask about guns when her kids were left with us to care for. She still left her kids with us. (we were the go-to family for when they had an emergency and needed to drop off the kids). Later on I arranged for the County Sheriff to come to the school and do a firearms safety presentation. It was attended by over half of the kids in the school. All of the feedback that I heard afterwards was positive, even from the anti-gun families. My justification was that we live in a rural community and if your kids were to come across a gun they need to know how to be safe. Period. I think all kids should have the same training, based on Eddie Eagle, in school. There is no pro- or anti- gun instruction, just "Stop. Don't touch. Get away. Find an adult." Pretty straightforward.
I wouldn't volunteer the information. Years ago, when my daughters where youngsters, I had a mother of a child who was coming to spend the night with my youngest ask if we had guns in the house. The answer was "no". It was none of her business, they were legally owned, and locked up safely with ammo stored separately. My daughters knew where the guns were kept but not the ammo and they had been taught gun safety. The area of the house they were in (the master bedroom) was off limits if either hubby or I were not in there. My daughters respected that even at a young age so there was no reason to worry that any child would get a hold of my guns.
I agree with Trio, I wouldn't say anything unless asked. I think a lot depends on how close of a relationship you have with these people.
I am fairly new to handguns (grew up around long guns) and play it pretty close to the chest on who (friends and family) I talk to about them.