Just take some extra large men's t-shirts and some scraps of fabric from other old shirts and you can make these adorable dresses.
There is a full tutorial on Make It and Love It. Go check it out and be sure to browse around this site a bit. There are lots of cute ideas to me found. She also has a cute Valentine shirt idea for boys that I think I might try too.
I love the sweet little ruffled hearts and so do my girls. The only change that I would make would be to make the ruffled heart about 1 1/2" bigger on all sides than the cut out heart shape. I only cut it about 3/4" bigger and I wish I had a little more wiggle room. Other than that I would follow the tutorial exactly.
The following photos are what happens when I tell my girls to pose however they want. :)