As you sew around the bottom, you will want to make sure the button plackets are pinned neatly together at the bottom.
Next, gather the top of the shirt.
Now we are going to make the band at the top of the shirt. Sew your 11" and 16" pieces together, creating one long strip. Iron it in half lengthwise, right side out, so that you have one long piece measuring 26.5" by 2.25". Take your 12 inch elastic piece and place it in the longer section of your strip just over the seam line (see picture). Attach a safety pin on the other end. The elastic should be about 1/2" under the ironed fold you created earlier.
Pin the elastic in place. Take one end of your unfolded strip and match it to the other end of the strip, creating a circle. Pin together and stitch from one end to just past your fold line, thus leaving a space for the end of the elastic to come through later. Fold your strip back together and topstitch over the top of your first sideseam, sewing the elastic in place. Baste the bottom of your strip together 1/4 inch from the bottom, making sure the elastic stays loose.
Take the main shirt portion and even out the gathers to match the tube you have created. Pin the bottom of the band to the top of the shirt, right sides together, matching the sideseams. The elastic opening should be facing out when the top is pinned to the band. Stitch into place using a 1/2" seam.
Finish your seam with a serger or zigzag stitch. Flip the band piece out and topstich along the bottom of the band, again making sure you do not sew your elastic piece.
Now pin the foldover elastic into place. Each piece should be approximately 2" from the center front and back (again, for a 5 year old, perhaps a little less for smaller sizes), with 1/2" below the top of the band.
Topstitch 1/4" from the top of the band, catching the foldover elastic pieces as you go. You may want to backstitch across each elastic piece one time as well, to make sure the straps hold.
Thread your elastic piece through the back of your band, using the safety pin. Pin it into place just past the side seam, inside the casing. Neatly pin your opening closed and topstitch across the outside of the sideseam, catching the elastic and sewing your opening closed all in one step (a little tricky, but it won't show- so it doesn't have to be perfect!).
Now, if you want you can change the buttons. It doesn't matter the size of the buttons, as you will not be using the buttonholes. Just cut the old buttons off and sew your new buttons through all the layers over the buttonholes.
To adjust for size:
This tutorial is based upon the size of my five year old, who is on the skinny side. Her chest measurement is around 21". To adjust for different chest measurements, take the measurement and divide by 2. Add 1/2 inch. This will be the length of your first strip piece. Then take the same number and mulitiply by 1.5. This will be the length of the second strip piece. The width of 4.5" can be used on all sizes. The elastic should be about 1/2 inch longer than the shorter strip piece.
*** Because little people usually have about the same size waist as they do chest, this top can also be used as a skirt, tucking the straps into the waistband. Or, just leave off the straps if you only want a skirt!
And there you have it! All done.