I 've made so many of these T-shirt dresses while my daughter was growing up. There are a ton of tutorials to learn how to do this easy and quick sewing project. Here is the latest version that I made for some special ladies to wear to school on Valentine's Day. I foo fooed it up a bit by double layering it, adding ribbon & lace on the top layer. The double layer fabric was so thick that I had to do the zig zag over a string method for gathering the skirt. I also added a side bow with a heart shaped button in the center. For the top, I referred to one of my pins from this site http://youretoocrafty.blogspot.com/2011/10/we-are-celebrating.html.
What I like about the T-shirt dresses is that you can wear a dress that is festive and fun but not have it to fancy and be comfortable enough to run and play.
Happy Valentine's Day!