So at what age did you explain to your kids what a church is?
"It's a place that some people go sometimes."
LOL...well, that's a good start, right?!!
I did not and do not wait for them to bring it up. I guess I figured that a better defense is an offense. Give them the knowledge first so that when they are faced with it, they will have a framework for whatever they see or hear.
I have read many many myth stories to my children, ancient myths and current myths from the different religions. Always within the context of "reading myth stories. So always in the context of "This is what some people believe/believed before we got science to explain the natural world." From there, they were always prepared for the cutsie way that religions like to present themselves to children.
Also, I'm afraid that one way my kids first learned more in depth about Catholicism, we were taking a walk and stumbled upon stations of the cross. I was explaining what they were and they were interested, so we walked the "stations" and I explained each station to them (as a recovering Catholic myself). That story ALONE is enough to turn anyone away from the church. So gruesome and ugly.