Haimy,
The question has tremendous ramifications; before we suggest to people to allow chillul Shabbos, Z’nus, tarfus, V’chol dovor assur into their homes in order to maintain a positive relationship with their non-religious child
I don’t accept the premise of this statement. I think it is incumbent on parents to try and maintain a positive relationship with their child. At the same time, as managers of the home, the parents have a right to set rules and expectations for what happens under their roof. Having different beliefs does not make it ok for a child to defy rules or severely disrupt the home.
“This is a kosher kitchen. Please do not bring that food in here.”
“This home is Shabbos observant. We expect everyone here to put cell phones away. It can be used later tonight after havdala.”
Nothing unloving or even judgmental in those statements.