looks like you've got a project already but i wanted to add some too.
in school, i studied self expression through art more so than the manual dexterity or social skills an art project might provide, so, along those lines, i would try something like abstract sculpture. pick up a bunch of different shaped wood pieces (available around here at many craft stores), or a pack of assorted sized/shapes of balsa wood, some primary paint colors so they can use them straight up or mix their own, and some extra stuff for visual and tactile stimulation…cotton balls, foam shapes, yarn and string, dry pasta, sand paper cut into shapes…. checking out the local dollar store is great for ideas here, too…you can get a bunch of plastic matchbox cars, bendy straws, little army men, doll clothes, puzzles pieces. the list is endless.
a hot glue gun puts it all together and viola! your own little lost boy-cristos
as far as just regular woodworking projects.
you could pre-drill a flat board to hold pegs and make a rack to hold...whatever...coats, keys, BBQ tools
four pre-cut sides, a bottom and a staple gun and they could make an open-top box for pens or mail.
some pre-cut teardrop shapes, paint, a glue gun and a few small tree branches and some twine to hold it all together and they could assemble a bouquet of flowers (maybe for mother's day?)
that's all i got right now.
whatever you do woth them, it sounds like you and your boys are gonna have lots of fun...i'm jealous.