Knit romper...that is is a boring name for such a fun little outfit! I got the fabric at Hobby Lobby, it was a remnant so I got it for like $4 or so. I'm not too experienced working with knits, but this fabric was not a problem at all to sew with. I love that its super comfy but stylish. Also it's girly but different.
This was the pattern I used. Got this for $3 at Walmart! I altered the pattern a bit to make it a romper instead of a dress. I love the romper look, but the fabric around the crotch ended up being really weak because I didn't take care to strengthen it but oh well, it has held up okay so far.