My dd and ds do that as well both 4 and 2. ds isnt too bad now he knows its a naughty word but if he sees something really exciting he will go Ohh F(**** mum.
Its my fault and hubbies fault as we used to say it not intentionally, but at the heat of the moment etc.
Now ds does it this sweet little girl sitting in the pram and she is going ohh F***.
I do tend to ignore it now and havent said a bad word myself for a lonnngggg time!!
Each time she sais it now i said oh no say Wow so she is replacing all the F moments with wow, i just say it and i think its working....slowly!
it is hard though hubby is a mechanic so when the boys are out in the shed having a get together they go out there and hear all the man talk.
My biggest mistake was laughing which i guess it completely normal at first as you think omg what do i do but now the Wow factor is the best so far for me!
It will pass i am sure. Good luck