That child sounds like a toxic narcissist. What's their star sign? I'll bet I already know without asking.

If I were you, I would have explained that life is not fair, and if they start gatekeeping competitions, then all they are really getting are participation trophies and are basically "whatabouting" actual science fairs. It's almost like they don't believe in fairness at all, despite being at a fair. I'll bet if you look deep enough into that kids homework, you will find out they learned where Ruzzia is on a map, which makes them suspect at best.

How are you going to learn to make a proper volcano whilst gatekeeping the adults in the room who make the tough decisions like what whether to use plaster of Paris or papier mache? They're too young and idealistic to get their volcanoes to be accepted by the broad public anyway. Best get behind the person with experience. You can brainstorm better volcanoes after we win the most important science fair in our lifetime.