A few months ago I saw a girl on the playground wearing a shirt similar to this one. I thought the idea of the ruffly sides was really cute. Unfortunately, since I did not know the child I could not just walk up to her and take a closer look, or even take a picture, so I made this from my memory. I actually really don't remember what the shirt looked like, but I think this one turned out kind of cute :)
Anyway, it is really easy to make, you just need to alter a normal shirt pattern a bit. I will briefly talk you through it, but I don't have to explain much, if you know how to make a regular t-shirt...
You can download my pattern here. It is size 3-4T. If you need another size, use my pattern as a guide...
Cut one back shirt piece and one front shirt piece, 2 flap pieces (on the fold) and one long strip, about 2" wide.
Cut neck band to the length you need it and sew in neckband. If you are not sure how try this tutorial or this tutorial.
Sew on the arm bands (the same way you did the neckband).
Ruffle the flap on the rounded part. Pin to the rounded part of shirt and sew on.