Still a little chilly outside?
I used sweatshirt fabric for this dress with knit for the sleeves. Cut and sew the dress as directed until you get to the sleeve casings.
To make the sleeves, find a shirt that fits the way you like on your little one. Find the length that will be needed to fit inside the sleeve to the wrist. Place a pin on the shirt at the height you want your sleeves. To make a sleeve pattern from your existing shirt, lay the shirt down flat, with the underarm seam out to one side. Take a piece of paper and fold it in half. Place the paper under the sleeve with the fold exactly where the fold of the sleeve is. Trace the outline of the sleeve with a 1/2 inch seam allowance on the top and side. You will also need a 1/2 at the bottom if you want to hem the sleeve. (I didn't hem the sleeves-- knit doesn't fray!) Cut your paper pattern out on the line and unfold. There you go!
The sleeves will need to be made out of stretch fabric for this to work.
Sew the sleeve together down the underarm seam. Place the wrong side of the sleeve on the inside of the dress sleeve, about 3/4 of an inch above the bottom edge of the dress sleeve. The sleeve is going to be much smaller than the dress sleeve. Pin the sleeve seam to the dress sleeve seam and the middle of the top of the sleeve to the middle of the top of the dress.
As you stitch the casing edge, also stitch the additional sleeve into the casing by stretching the sleeve as you sew to match the seams and midpoints.