True to stereotype, my boys are more into building stuff (and knocking it down) than they are into settling into a calm craft project. I tricked them this morning by having them build a city with some simple building stamps I made for them. They were into it for about 15 minutes which was the most I can hope for. I think the big 20 minute attention span mark comes at 4-years-old:-)
linoleum block (I used Speedball brand, you can find these at most art supply stores)
wood carving tools set or linoleum carving tool with various attachments
Using a rotary cutter I made simple triangles, squares and rectangles in various shapes. You can make one deep cut with the rotary cutter and then simply bend and break the linoleum for a clean edge. You can also use your carving tools to scoop out a design. Experiment, have fun and keep it simple if it's your first stamp-making project.