Mr. and Mrs. Simon are Margaret's parents, and as far as parents go, they're pretty cool. The only real drama these two bring to the book happened before Margaret was even born, and their response to that drama from way back then is measured and reasonable: they decide not to push any religion on Margaret, and instead to let her choose for herself when she's older. Here is how Margaret explains to her friends what went down when her parents decided to get married:
"Well, my mother's parents, who live in Ohio, told her they didn't want a Jewish son-in-law. If she wanted to ruin her life that was her business. But they would never accept my father for her husband." (5.66)
And so they've never been part of Margaret's life either. With no siblings in the picture, this means that Mr. and Mrs. Simon form a pretty tight-knit unit with Margaret. But here's the thing: Remember their decision not to put pressure on Margaret about religion because of their own bad experiences? That was fine and dandy in New York when religion wasn't really on Margaret's radar, but now she's facing some major questions and doesn't feel like she can share them with her parents. So much so, in fact, that she doesn't even tell that that she talks to God. As Margaret explains:
My parents don't know I actually talk to God. I mean, if I told them they'd think I was some kind of religious fanatic or something. So I keep it very private. I can talk to him without moving my lips if I have to. My mother says God is a nice idea. He belongs to everybody. (2.132)
So while there's nothing bad about Mr. and Mrs. Simon as parents, Margaret doesn't feel like she can turn to them as she tries to untangle the impossible web of religious options. Heck—she doesn't even feel like she can tell them she talks to God. And that means that she keeps a very big and important part of her life from them. Do you think she really needs to? Is it possible she's just misreading them?
The cool thing about this inability to come together over religion, though, is that it kind of keeps God supreme for Margaret. There are plenty of things she can talk to her parents about, after all, but God is the only one she tells everything to.