When children are teething they sometimes chew on their cuffs. The cuffs are a handy teething ring, but it is hard on the fabric. Your can easily make new replacement cuffs from 3-inch rib-knit trim. If you cannot find fabric that matches your sweatshirt, you can make your own trim from rib-knit fabric. After you replace the cuffs on the shirt, make an extra pair of cuffs for your child to wear over the new cuffs. He can chew on the over-cuffs until his teeth emerge.
Measure the distance around the bottom of the sleeve. Add 1/2 inch to this measurement.
Stretch the rib-knit trim and while it is stretched cut a length of trim equal to the measurement you calculated. If you are making you own trim, cut a 6-inch strip and fold it in half lengthwise so it is 3 inches wide. The ribs of the fabric should be perpendicular to the fold.
Fold the trim in half widthwise and sew the ends together with a wide zigzag stitch. This forms the cuff.
Stretch the cuff as you pin it to the sleeve. The fold should point up on the outside of the sleeve and the raw edges should be even.
Sew the cuff to the sleeve with a wide zigzag stitch, keeping the cuff stretched as you sew. Stretching the cuff insures the cuff will gather the end of the sleeve together when you release the tension.
Fold the cuff down and be sure the seam allowance is pointing back toward the sleeve.
Top stitch by stitching the seam allowance to the sleeve fabric 1/8 inch above the seam with a straight stitch. Make the second cuff in the same way.
Make a removable "overcuff" to protect a shirt's cuff by finishing the raw edge with a zigzag stitch instead of sewing it to a sleeve.