This is what I did/used for Maya's dress. She is 4 but she is a small 4.
I used quilting cotton. This dress needs two rectangles for the main part. The width of each rectangle should be about 20 inches and the length should be the length you want the dress to be plus 5 inches (measure from about the armpit to where you want the dress to end and then add five inches) I also cut two pieces about 4x 14 for straps. You could also use ribbon if you prefer. You will need some elastic for the top. I used 3/4 inch and it worked well.
Sew the two rectangles together along both sides (right sides together). Press and then zig-zag, pink, or serge your edges. I don't own a serger yet so I pink or zig zag. In all honesty, I don't always press my seams either. In an ideal situation I would have the ironing board out right next to my sewing machine but I am usually grabbing a few minutes to sew here and there - mostly after the kids go to sleep and before I pass out. I often finger press. I also sew at the toddler table in the family room. It is not a great set up but it is the only downstairs spot where I can have a flat surface and an electrical outlet.
With the dress inside out, fold and press the top of the dress body down about 1/2 and inch. Then fold it over about 2 inches. You are going to make a casing with a ruffle or paper bag style top. Pin your fabric and sew one line all the way around about 1 inch from the top of the dress. You can sew all the way around without stopping because you are not going to thread the elastic through this seam. Then sew around again at the edge of your folded fabric. This time you need to leave an opening to feed the elastic through. I backstich at least 3 stitches at the start and end of a line of stitches. Then measure you child's chest in order to determine how long your elastic should be. Instead of using a tape measure I would just take the elastic and wrap it around to see what length makes a snug fit. Be sure to add a little extra length to make up for the parts you will sew together. If you are making more than one dress just measure the elastic after you have cut it and jot it down the length so you know for future reference. Attach a safety pin to one end of the elastic to feed it through your casing. Zig zag stitch the two ends of your elastic together back and forth a few times and then go back and finish sewing up the casing.
Take the two strap pieces and fold them in half lengthwise. Sew down the long side. Fold back out to the right side and press. On the short ends, fold the fabric under about 1/4 inch and topstich around the entire strap for a nice finished look. Pin the straps to the dress and sew along the TOP line of you casing to attach the strap. HINT: Sew with the dress right side out so you can more easily blend your strap stitches in with your casing stitch line.