I say let the kids joke. You know to them it only makes the joke funnier if you get upset over it. Kids are kids, jokes are jokes. Anything you try and do to stop it will only make it worse for you and more enjoyable for them. Besides 90% of the the time it’s used, it makes no sense at all so oh well. I don’t like to get offended over much of anything especially jokes. I’m not saying I condone the use of the phrase but there’s nothing we can do about it so just take it with a grain of salt. They’ll grow out of it soon enough.