You would just measure how long you want it, and add a little for hem allowance on bottom. Measure the child around for the width and double that. Sew up the side. So you start with your tube of fabric. You can add a band of coordinating fabric at the bottom like I did. With the seam on the left side iron flat.
Using this armhole guide cut out the groove for arms. This is just an example, I always just wing it. It's about 2" across and 2" down, this will work for most sizes.Fold twice (1/4"-1/2") and hem bottom of dress, I just top stitched.
With coordinating fabric sew a casing for the elastic between the armholes at top.
Madeline's elastic was 6" long.
You will need to measure the child from underarm to underarm to get this measurement.
Run elastic in casing and when you get it about half way sew the elastic down at opening, then run it all the way and sew down the other side. Do this on front and back.
Finish the armholes with about 36" of bias binding on each side.
If you have any questions about this PLEASE just ask.