Get there early in the day! Last year we went on Saturday and liked it so much we returned on Sunday.
Open House - NASA Jet Propulsion Laboratory
Bad news update: this event has been cancelled for June of 2013. Hopefully they'll reschedule it.
JPL suspends June open house, cites U.S. budget woes - latimes.com
I suggest charging $10 for parking and $10 for adult admission. (Let kids in for free and admission optional--donation suggested--for adults accompanied with children. One adult admission per kid. Sell/reserve adult tickets in advance and limit the number, then announce that the event is sold out after reaching that cap.)
This event needs to take place in 2013. We're curious about Curiosity on Mars.