I Need to Vent

So I just found out at the last minute that I am in charge of instructing two kids' art classes at the art store I work at tomorrow. I have no experience in teaching or children, and the managers never bothered to ask me this, either. Apparently, I'm substituting for our usual instructor. She can't do it because her child's b-day party is tomorrow. I don't know what we're making or how to make it, how many children are coming per class, if they'll bring the art supplies, if the store provides them, or if I have to buy them (I hope not, that's expensive and I've seen our knitting instructor buy class supplies). I've been scheduled to do this since last Thursday but no one bothered to tell me so that I'd be prepared for all of this. I asked questions and for advice but I was just told to come in early tomorrow to read the instructions and practice making the projects, but the problem is that they give the usual instructor all the answers and instructions for the projects a week ahead so that she can practice making it before teaching the kids. This feels very unfair to me, and perhaps I'm overreacting but I feel screwed over. Would it have hurt so much to ask me if I could do it, or if I even have the skills for it? One of the managers said she'll help out if needed but she's kinda new to our store. But at the same time, she's a grandmother so maybe she'd be a great help when I need her. I don't know what to expect at all and I'm panicking! What do I do???