When my wife became a baalas teshuva and took on kosher, she was advised not to inform her parents of this switch and to continue eating treif in their home- i.e.- Kashrus is a big enough deal, but include the parents also?!
We continue to buy only Kosher, but when at my in-laws…as long as it looks kosher. Our children accept that we only eat McDonald’s burger’s and Tyson poultry at Grandpa’s…except once, at the grocery, when my 4-year old asked me to pretend I’m Grandpa…