Kid daycare right? Wear clothing that looks nice but that you can move in and don't mind getting dirt/stains on so you can be right in the middle of the action if needed.
When I interviewed for dog jobs I needed similarly flexible attire, so I went with khakis and a nice polo shirt. Consider length of the shirt, too, since you don't want to show too much skin when bending over!
Also, I second the smile!