I guess you are buying "The art and science.." used, as it is long out of print. Last time I checked it was really expensive on the used market. Sailrite.com also make some decent leaflets on how to manually lay out and cut sails.

I believe your battens will be very stiff (too stiff, possibly), if you add carbon or even glass to the cedar. Cedar is much stiffer in itself than foam, add some glass and it becomes really stiff. 4mm spruce with a light layer of glass on each side is strong and stiff enough for hull construction, just to put it into perspective. I would have tested it with one batten first..

At age 9, 11 and 12, might I suggest some bathing, capsize and righting exercise before you go wild (sailing)? Just to make sure you know how and are able to right the boat. Can be a real boost for your kids confidence on the boat and add to their perception of it.

Good luck with the project! Sailmaking is not rocket science, just a craft like any other. When you get around to installing the luff rope, let me know so I can tell you what we did wrong the first time.
Please post some pics of the process and the finished result when you are done <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />