Little boy refuses to say a "bad word"

A little boy learning how to say words with his flash cards can't bring himself to say a "bad word".
But he is AS innocent AS you can get. 
The "bad word" he mistook is similar to another "naughty" sounding word, but despite his mum saying it was fine, he still refused to say it.
"I can't say that," the little boy tells his mother over and over.