Took a mens polo shirt and laid an existing tank top on top of it, placing it on the bottom (but making it 2 inches longer than the existing one) of the polo shirt so I could use the existing hem. I cut around the tank top making it slightly larger to allow for hems but also to allow for the growth of the 3 year old.
Then I sewed up the shoulders and side seams (right sides together) and then used (cut up) a yellow ribbed tank top and made the ribbing for the neck and arm holes. There you have it. Twenty minutes to a new tank top!