What The Heck?!

I am standing at the stove top preparing dinner. Young Ava comes up to me with innocent wide-eyes and asks "What the heck you doin', Mommy?". Pardon me?!

It took me aback. And I tried not to smirk as I asked her to repeat what she said (since I didn't believe my ears the first time). I guess heck isn't such a bad word but how do we know what is or is not acceptable for a 2 (almost 3) year old to utter? How do you gauge this?