It comes to most parents, and has come to us too. Our darling angelic child (erhem) has learned the 'f' word (yes, the one you are thinking of). I blame Simon and he, while admitting guilt, feels I am not without blame either. She says it with conviction and perfect, deliberate annunciation,and usually in an appropriate context. It can be very hard not to laugh, but as ignoring it has not erased from Em's vocabulary, I have had to explain that it's not a nice word and we don't say that. It's not entirely worked as yet..!
Some classic instances:
In bed, after I had finished reading stories and was tucking her in.
Em: F**k, f**k, f**k. I'm just saying f**k to my sheets Mummy.
In the bath the other night, apropos of nothing.
Em: F**k. F**k. F**k. Spagiddy*. Humbug.
That was one of the times it was very hard not to laugh! (*Spagiddy, emphasis on the 'spag', is how Ember says spaghetti).
Today, in a (mercifully quiet) cafe, after I spilled her fluffy.
Em: Oh f**k.
Em: We don't say f**k, aye Mummy. We don't say f**k. I won't say 'f**k' ok Mummy? I won't say 'f**k'.
Em: It's not nice to say f**k, aye Mummy.
Me: Well stop saying it then!
Bless them. Hmm.