If your child has blue jeans that have ripped-out knees, but still fit, there is a way you can repair them so they are no longer damaged but rather become "cool" and "stylish." All you need is a bandana and some heat-activated seaming tape, an alternative to sewing that eliminates unsightly stitch lines on the front side of the blue jeans.
Turn the blue jeans inside out, then lay them down on an ironing board or any other large flat work surface. Draw a box around the area to be patched using a fabric marker and a ruler. Measure the box both vertically and horizontally with a ruler. These measurements will let you know how large your bandana fabric needs to be.
Lay the bandana down flat on your work surface and, using a ruler as a straightedge, draw a box on the fabric like you did on the jeans. Cut along the lines of the box with fabric scissors.
Lay the cut piece of bandana fabric with the pattern side down on your work surface. Cut four pieces of heat-activated seam tape, one piece for each side. Lay the seam tape on the edge of the fabric so the outer edge of the tape is even with the outer edge of the fabric. The pieces should overlap at the corners. Press the seam tape down gently to ensure it stays in place when you pick up the bandana fabric.
Lay the patch over the box on the blue jeans, both of which are the same size, making sure all edges line up and are even with each other. Press a hot clothes iron down onto the patch for one minute, slowly rubbing it over the edges of the patch where the heat-activated tape has been placed. Do this for all tears or slits in any part of the blue jeans.
Turn the blue jeans right side out and inspect them. Care for the jeans the same way you always have. No special precautions need to be taken to prevent the patch from falling off -- the heat tape is very adherent when used properly.
Measure carefully, making sure the edges of the box extend at least 1 inch beyond the edges of the damage of the blue jeans.
Use care and caution while using a hot clothes iron to avoid burns to yourself or the fabric.