I agree with Jake 100%.
Coming out of monohulls my first cat was an old Hobie 18. I raced it with my 6 yr. old son. They are built like a tank, wide, stable, lots of volume in the hulls for more than 1 crew (I often sailed it with my 3 kids on it) and I often sailed it alone in 15-20 knots on my lake. (not out in the big ocean, that would be stupid)
Mine had a rolling jib which was nice when the big thunderstorms would come by with huge 30-40knot wind. I'd just roll up the jib, pull up the boards, let the traveler out and haul azz for my beach, drop the main and run for the house!
I only paid $750 for it (12 years ago), but it was sitting in a guys back yard for 5 years and needed some work, so I fixed it up, sailed it for two years and then sold it for $2,500 to get a "Racing Boat" (Nacra 5.5).
If you can find a good old TheMightyHobie18, you won't lose money and you will have a great time with all the kids on board. The Hobie 16 doesn't have a whole lot of volume in the hulls for more than 2 adults, or you and 2 kids, and is prone to pitch you over the handlebars when you get going fast downwind, so if you go that route, be careful and learn how to right it. I recomend Rick White's excellent book, "Catamaran Racing for the 90's" available on this web site in the store link above, it fully explains how to right a cat as well as lots of other great speed stuff.
http://www.youtube.com/watch?v=gWBvVedo9QU Hey Rick, about time for a re-write of the book, yes? To include all the Formula Spinnaker classes and the new ideas coming out of the A cats. Maybe we can get Randy to write it?