Don't forget the buckets of water. What signigficance do they hold?
It's been years since I last saw Fantasia, but I think the wizard left Mickey to clean up, hence the broom. Mickey decides to take the easy way out, borrows the wizard's hat to conjure up some helpers, and finds himself in over his head - literally, as the water the brooms are fetching continues to rise.
The lesson, I think, was not to mess with powerful things that you may not be able to control. Not a bad message, IMO.