Since talking it out is not on the table any more, I think there are two ways to think about this problem: as a son and as a father.
As a son, you are dealing with an age-old challenge. The proper way to disagree with parents is a common conversation in Jewish texts. In the Mishneh Torah, we are told that the way to tell a parent that they are sinning is not by directly informing them, but by inviting them to look up the law with you. This is strategic. A person who spent your formative years being an authority figure in your life does not easily relinquish the role. So I suggest you take up a little informal research project with your mom, and together seek wisdom and advice from the outside world that pertains to your conundrum — maybe from Jewish sources, and maybe not. Writing to the Seesaw was a great first step.
We spend a lot of our lives waiting for our parents to be the parents we want them to be. It does not happen. And if you wait, you will only be disappointed. Her faith is not on the table here, and neither is her practice. Using the strategy of the Mishneh Torah here is likely the best. It will allow both of you to preserve her dignity, and might even get her to see how she is meddling in your family life without you having to tell her directly.(Again.)
As a father, it is also important that you have a good talk about religion with your children and help them understand how they feel about it, and then support them in their choices. This way, when their grandmother begins her annual conversation about Sabbath observance, they have a better sense of where they stand with all of this. Also, as they get older, your children may make different religious choices from their parents, just as you have done. I imagine you are prepared for the possibility that they will want to express their Jewish roots less than they do now, but their exposure to their grandmother’s level of observance may also lead them down a more religious path, if it provides solace they want. Opening a conversation with them about religious choices now might help you avoid your current problem with your mother recurring in the next generation.
Benjamin Kamine is a New York-based stage director, primarily focused on new works. His recent credits include productions at The Goodman Theatre in Chicago, The Flea Theater in New York, and development work with companies all over the country. He was a 2014-15 Fellow at LABA: A Laboratory for Jewish Culture, and is currently working to expand his knowledge of Jewish texts.