As a cashier myself, this is so true. I say no problem all the time, and no one else has a problem. You sound just as nice when you say "no problem" vs. "youre welcome" ; Agreed, I say "No problem" to people all the time.
If you see or hear signs or symptoms of child abuse report it right away. Approximately 5 children die everyday form abuse. You never know if it will be too late. Do the right thing and stand up for these helpless victims.