The fact your seeking feedback suggests no, but it was certainly a bad move, both as a DM and as an uncle. Punishing anyone, though especially children, without explaining why is mean. You have a responsibility to clearly communicate problems with others as an authority figure at the table and in their life. I don't necessarily think the punishment was unreasonable, but if it's not explained to them, it just comes across as arbitrary and vindictive.

Imo, the best way to handle issues like that is to set the rules and consequences, making them clear to everyone, and to be consistent in their application. Letting people off or being vindictive will just exacerbate things.