Stepchildren and language

[PG2018]

I have to dsc 4 and 6, twice this weekend they used the word fart. The first time I was looking after them on my own so told them it wasn't a word we use. Today they used the word again, DP didn't seem to bat an eye lid (although did later agree with me). I think if they were older I wouldn't like it but would accept it more. Is this inappropriate or am I being a prude?

There is a current 'earworm' of bumbum chicken suck my toes happening in our house at the moment. Fart is on the wrong side of crude but not the end of the world. Perhaps when they say it you can mirror back the word you would prefer they used?

I don't love hearing small kids calling it a fart, and mine used another, more twee word. But the eldest uses "fart" sometimes now, and I don't make a fuss. She hears me saying much, much worse.