If you can keep the mast up and keep the boat on a beach a Prindle 16 is just about perfect as a first catamaran. The best first sailboat may be a Laser dinghy though. A Laser would be less intimidating when your daughters are sailing alone for the first few times and you would not have to worry about them being able to right the boat on their own.
Once they can sail - get them the Prindle.

I got my first Prindle 16 when I was about 15 - and loved it. If mishandled by an invincible teen a Prindle 16 can in fact pitchpole violently. I proved it singlehanding in a good breeze (once). But then boys will be boys and girls will be...smarter.

The Prindle can carry 3 kids and a cooler of soft drinks and sandwiches. It is beachable and the kick-up rudders are robust. The external hull to deck joint and narrow rockered hull bottoms make it easy to pull up on the beach without wheels. I would recommend the mainsail with reef points - had to use them a couple times when the wind picked up and I was miles from home. Would also recommend a cell phone in a waterproof bag (ziplocks do not work) and a GPS. That way they can call home and let you know everything is OK - and tell you where they are.

Another good choice is the old Supercat 17 if you can actually find one. I think they draw slightly less than the Prindle and have more hull volume. They also have no boom. I have never sailed one from a beach through.

Definitely get them the Prindle 16 once they can sail. They will not get bored and beg you for a waverunner. Prindle 16s ought to run $1200 to $1600. Maybe the one in the paper has extra equipment.

-colin