Kids have to know, including the example set by their parents, that we exist to serve Hashem properly. Once that foundation is established and everything you allow or forbid (for yourself as well) is viewed through that lens, then it becomes much easier to do and require of your kids what is right even if other people do differently.
The answer to “but everyone does it” is, then, that they are wrong but they don’t know any better, and we will try to do only what is right, not follow other people in doing what is wrong.
This applies to everything from outright issurim to gray-area issues of sensitivity.