Buy a cheap plastic tarp and a big bag full of black (uv resistant) zip cable ties. Fold the tarp in half. Take a knife and punch holes every 4-6" along the sides of the tarp, inside the rope but through the hems. Stick cable ties through the holes as you go and sew the "bag" shut with the cable ties all but the top. Start at the top corners and work down the sides to keep it aligned. Stuff the bag and then sew the top shut with the cable ties.
You'll lose about a foot and a half from the dimensions when you stuff it so a 6' x 10' tarp will make a nice, big 4-1/2' x 3-1/2 target for the boys to shoot at.
Much better than burlap. Won't rot, won't absorb moisture. I don't even hang mine, just lean it against a tree and shoot away. Keep it out of the sun and it will last for thousands and thousands of shots.
Cheap, probably less than $15 bucks. Light. Fast to make, maybe 20 min.