When I was a student, I did my work placement in a nursery and when I was working in the toddler/baby room, I worked with a child who was particularly intelligent and talkative for his age (I don't remember how old he was exactly but he was due to move up to the next room shortly so he was about 2 years-old). One day, I tried to open a particularly-hard-to-open bottle up to refill and when I finally got it open, I muttered "DAMMIT" and this child heard it...and promptly said "Dammit!". I literally turned around and started singing "YOU HEARD NOTHING~! NOTHING~!" and made him laugh instead.
I also worked with a 4 year-old who threw such a bad tantrum that he started shouting incomprehensible words at the staff member trying to calm him down...and it took us 2 minutes to realise he was telling her to "f*ck off" and the staff member wasn't offended...she just laughed it off. His mother, on the other hand, was understandably unimpressed with his behaviour when informed how his dad had gone.