Just had this conversation with DS2 age 3:
Me: now DS2, were you being silly again in the garden?
DS2: well I was saying fucker again because DS1 was teaching me how to say fucker and I said it was naughty to say fucker but he still kept saying fucker. And Grandma told me that it wasn't nice to use the word fucker so I'm not going to say fucker any more even if DS1 tells me to, as fucker isn't a good word...
All while I was wiping his backside too!!
I'm all for desensitising the word but I'm don't think it's losing it's power even when he uses it 10 times in one sentence!!! 
Quite funny tho'. But I'm off to give DS1 a roasting!! 