I came up with a cute onesie for my friend's little boy. Although really, this could be for a boy or a girl!
I used the actual strap from a play medal I had leftover from prizes for a scavenger hunt. And I printed out the "medal" on iron transfer paper, and ironed it on to muslin, then sewed it on to the onesie. You might be able to just iron it straight on to the onesie, but I think the muslin holds the color/image better than the onesie tshirt material.
I might have to do some "big boy" versions for Thing 1 and Thing 2.