Telling her off for throwing a knife wasn't unreasonable, that's never OK and needs an instant response.

it's just as unreasonable for a parent to force a child to be around a unnecessary loud and triggering noise, especially when the kid is clearly demonstrating they are terrified.

the child tried to intervene to stop it, but because they don't have the physical power their parent has, their only option was to scream and throw away the tool to stop the balloon from being popped.

the kid had basically no power to set their boundaries in an urgent situation, and did the only thing they could do. the adults, however, had power to choose better reactions than screaming at an overwhelmed child, shaming, and hitting them.