I have several of the plastic ones you mention and used them a lot with my own kids back when making popsicles out of koolaid was the thing to do - dating myself here I know. Even cola and rootbeer ones. And yes any juice worked too. I found they melted just as fast or faster than the store bought kind so there definitely for the older kids and outside. Mostly I let the parents worry about this kind of mess. If I do anything at daycare it is the mr freeze type you cut off and push up. The yogurt should be fine but be sure to leave room for expansion. If it was too hard when taken out is should soften as it sits for a bit.