Re: And she presses babys soft spot!
NO!!! Do not lock someone’s kid in the bathroom…and turning off the light on top of that. 
When I told my mom that my aunt had done that to me…my mom was very angry. My parents did not express their anger as they’re not the kind to create drama, but there’s no telling how this little girl’s mom will react if she were to find out that OP did that to her daughter. You also don’t know how kids will react…locking in the bathroom and turning off the lights may develop trauma or trigger a phobia…or the kid can slip, have an accident, etc. Explaining that to their parent will be awkward.
Limit the disciplining of other people’s kids to just verbal or physically removing them if they’re causing harm. But leave the hitting and locking up to their parents.