Pillowcase dresses are so cute. I'm sewing quite  a few this week. I have a new go to dress!
My dresses have a ruffle at the top and a band at the bottom. They also have a button and buttonhole elastic for growing room. I use a steam iron as much as a sewing machine during constuction of these dresses.

Here's how I do things:
Measure the target child from armpit to knee. This is your finished length (FL).

A little figuring will determine the length of fabric to cut for each dress.
The ruffle at the top and the seam at the band adds 2".
The only variable is the width of the elastic for the chest. Add the width of the elastic (WE) to the total.
FL + WE + 2" = cutting length
For example: Abby needs a dress that is 18" long. I am using 1/2" buttonhole elastic. So 18" + 1/2" + 2"= 20 1/2". I will cut a piece of fabric 20 1/2" long.

My dresses use the entire width of the fabric. Abby's dress is cut 20 1/2" by width of fabric (wof).
Now for the band. I like the looks of a 4" band. So I cut the band 9" by the width of fabric.

On to the sewing! Fold the dress right sides together matching the selvedges. Sew a 5/8" seam making sure to backstitch at the beginning and ending of the seam. Fold the band, and sew as for the dress. (NOTE: make sure the dress and the band are the same width. One seam may have to be a bitty bit bigger than the other.)

Press both seams open. Fold the band wrong sides together matching cut edges. Press flat. I use lots of steam, but that's just me.  Pin the band to the dress right sides together. Stitch using a  1/2" seam. Press seam towards the band. Press the band towards the dress. Sew a straight seam around the band, a generous 5/8".  (See pictures.) The raw edges of the dress and band are enclosed within this seam.
This makes sure that the dress won't ravel, and the hem will NEVER fall out.

Now for the top of the dress. I put buttonhole elastic and a button in my dresses.
Fold down, wrong sides together, the top edge of the dress 1/4". Press flat. To make the casing for the elastic, I fold the pressed edge down the width of my elastic and 3/4" more for the ruffle. Press this flat. No sewing yet.
At the seam, mark for a buttonhole near the bottom fold of the fabric. Sew a buttonhole that your elastic can fit through. Sew a button beside the buttonhole. I put my elastic under the button as I sew it on. (The elastic goes inside the casing with a safety pin attached. The pin will allow the elastic to be threaded through the casing once it is sewn.) Cut open the buttonhole. All this is done before sewing the casing for the elastic.

Pin the casing into place taking care not to catch the elastic. While pinning the casing, pin the ribbon ties also. Sew a straight seam around the dress at the pressed fold. Moving the elastic as needed, pin the top edge of the dress. Sew a 5/8" seam to create the ruffle. Thread the elastic through the rest of the casing and out of the buttonhole. Button the elastic on the button.