The toddler can't see it!
No wonder so many parents have mental health issues in this day and age! They're told that most reasonable forms of punishment are damaging to a child such as giving them time out causes attachment issues, raising your voice causes low self esteem ect... and then, when the child turns into a horrible, rude, and self centred teenager/adult, people blame the parents as they weren't strict enough or were lazy parents who couldnt be arsed to discipline.
I often mutter 'for fuck sake' when my 4 year old is saying mommmmmm for the 5th time in as many minutes. It helps me to stay calm with him whilst letting my emotions out privately. He does hear me say it so no damage done! Mind you, even if he did see me looking stressed, it would be good for him. Kids needs to know that their parents have feelings and needs and life isnt all about them.