I could see it being that a) she isn't physically stronger than other kids her age and b) she usually only puts a token amount of physical strength into her attacks.I think Kaya-chan can actually hurt humans, like other ghosts can, she just instinctively doesn't / doesn't know she can and doing it might be crossing the line that make her human 'facade' fall apart.
Getting motivated to hurt humans by the festival guy and those ghosts using her friends as vessels might be the trigger for the next arc.
It's called a kid, weirdo.Exorcist Loli