I built a Puddle duck and found it a perfect size for adult and child. Works well for two kids, and I've taken it on a week-long trip through the Boundary Waters with two adults, their personal gear and an equal share of the group's gear. It was the lightest boat to portage, but it was pretty low in the water for the first few days with about 600 pounds aboard. Paddling stern first with the paddler sitting backwards on the bow seat it works well as a solo canoe for fishing.
It is light enough for two 10-year-olds to carry and launch.

If you want to go smaller the Wee Lassie and Wee Laddie designs are based on boats built by Rushton in the late 19th century. In those pre-epoxy days these boats weighed under 25 pounds and easily held a good-sized fisherman and his fish. It appears to me that some of the modern adaptations have lowered the sides an inch or two. This may make the boats look sportier but it decreases the maximum carrying capacity. A shorter boat must sink lower to displace the same weight as a longer one. Look for a version with high sides, or add an inch or two to your forms with some scrap wood . Clark -Craft www.clarkcraft.com sells plans, kits and precut forms for this design. If you want a quick start go with the plans and forms.

Check Geodesic Aerolite boats www.gaboats.com for some ulltralight small boat designs. There is a solo canoe which will handle your weight but probably not hold two.

Several folded plywood boats including "sweetpea" and the "6-hour canoe"

Amazon.com should have the books on the last two boats as well as Gil Gilpatrick's book on strip built boats.

Consider a pirogue. Google Uncle John pirogue for a description and online building instructions, which are mostly intact. To get the rest you'll need top send a few $$$ While the usual plan calls for covering the pirogue frames with exterior plywood you could easily use redwood strips instead. Glass the outside. Leave the frames inside and just paint the interior. With two coats of epoxy. A pretty flat-bottomed canoe built quickly. With the flat bottom and sides you won't need to mill bead and cove edges on your strips, or hand bevel them. Square-edged strips fit perfectly. Rip 'em and start stripping it.

Hope these ideas help.

If you are anywhere near Indianapolis I have forms for the puddleduck and Wee Lassie. Trace 'em, or cut dupes with a router and patterncutting bit.

PGJ