2. She "doesn't seem to get" this, which I take to mean that she's still declaring her desire or intent to move in. (Not coincidentally - do you know who would have taken "no" for an answer? Someone who also would have respected your boundaries and not imposed herself on your child-rearing decisions.)
3. And you want a solution that won't hurt her feelings, which I take to mean that you want her to change her mind so you won't have to be the bad guys.
I'm a pragmatist, not a possessor of magic powers. There's no option here that's cost- or consequence-free.
That means the only good choice is the one that most honestly reflects your priorities. So far, you've cast this as a decision between your interests and your mother's. It might help to clarify things if instead you cast it as a decision that must be in the best interests of your son.
Would he benefit more from having your mom there, from seeing adults learn to reconcile different beliefs and practices, from having another adult to care for him, from seeing you treat an elder with compassion even though it means you and your family must make sacrifices?
Or would he benefit more from living in a consistent and harmonious environment, learning from parents who aren't hostage to their own conflict-avoidant tendencies, and who are mindful that adhering to their beliefs often leads to unpopular choices?
Wanting to make everyone happy, or wanting an awkward situation to go away, creates prime conditions for wishful thinking. Chances are you know enough about yourself, your husband, your mother, and your son to know who needs to be protected from whom, and why; now's the time to be truthful with yourself about these dynamics, brutally so, and to make the difficult call.
Q: I have a very dear Christian friend. We have attended church and study groups together for years. I am no longer a believer and would like to stop going to church, but I know she would perceive that as a sin and end our friendship. I can understand that because her allegiance is to God. Do you see a solution?
A: From the godless side, I see this: Either you admit your change of heart and be brave about any consequences, or you maintain your church facade to trick a woman of faith that a central tenet of your friendship remains intact, simply because you don't want to risk losing the pleasure of her companionship (and/or her approval).
In other words, there's hardly a decision to be made.
Since this friend apparently regards her church as a guiding authority in her life, you can also approach your question from the God side, and talk to a pastor you've found to be particularly accessible. Yes, I know, it'll be a bucket of giggles to share your non-belief with a church leader, but part of any faith involves living in a world where not everyone agrees. Since you seem distraught over how your friend will respond, why not brush up on the template she'll most likely use?
E-mail Carolyn Hax at firstname.lastname@example.org, follow her on Facebook at www.facebook.com/carolyn.hax or chat with her online at noon Eastern time each Friday at www.washingtonpost.com.