The cauldrons at Emily's Harry Potter party were ice cream gallon buckets. Apparently, we consume vast amounts of ice cream as a family because we found quite a few of these lying around the house. We spray-painted them black, but if I were doing it again, I'd paint them instead - particularly the handles, which didn't take the aerosol paint well.
We considered several options for the obligatory pet that the wizard students adopt at Hogwarts. One was an owl, for which Emily wanted to have a helium-filled balloon for each guest, which they'd then decorate with paper stick-ons to make a floating owl. This didn't pan out for several reasons: the cost of a helium tank, the likelihood of half-finished owls escaping and floating up into the sky, the stress of accidentally popping an owl while poking at it with a Sharpie, and so on.
Eventually, Emily suggested these mini rats from IKEA.
One of the more valuable results of having your child plan their own party is their learning first hand the feasibility of certain ideas. Often, cost is a screening factor early in the decision-making process, but sometimes time is, or equity-among-guests. There is something both conclusive and empowering about the child themselves realizing, "okay, this won't work. I need to have something cheaper/smaller/easier to mass-produce" and then doing the necessary research to generate alternatives.
Here are the mini-cauldrons that Kate made for party prizes. She used polymer clay. The round things atop the cauldron are bubbles, and there were supposed to be far more bubbles per cauldron than appear in these photos (the rest sadly fell off prior to the photoshoot).