I did the same when my girls were little. Cut off the sleeves and recut the armholes and then recut the sleeves to go back in. Don't touch the neckline or hem. Seems up sides under arm if too big. A-line it. Then I let them wear the cotton tights under it. There were great!
Upcycled Recycled Repurposed Sweater Leg Warmers Ikat Charcoal Teal Taupe Ruby. $24.00, via Etsy.