I just went thru this with my 8-year old in L.A....write a simple message about where people are going, and pin it to the wire netting fence near the main gate.
That's a great plan. (Do kids still write on paper nowadays?

)
Another option that bypasses the whole communications thing is just tell people that chances are you won't be able to get in touch. If a major quake strikes, everyone wants to know instantly if other people are OK.
I just tell certain loved ones that depending on what time it happens and how much damage there might be, if there is every a major quake, it will probably be tomorrow by the time I walk home from work so don't worry if you don't hear from me for a while. Easier said than done, but at least they know that's my plan.