First of all, you do a great job! Don’t let one mistake ruin your fun in making them! No matter what you do in life, there is bound to be a disappointment, but we learn and move on! :)
As the first poster said, when you do a tiered cake, even small ones, you should put supporting dowel rods, and even cake plates if needed, just like you would a wedding cake. Anytime there is a weight on top of a cake, you run the risk of it totally collapsing.
As for the fondant sliding, did you use a lot of buttercream or other crumb coating under the fondant? Maybe that could have caused it once it came to room temperature? (just a guess)
Either way, keep doing what you’re doing! You’re doing a great job! :)