My daughter was watching “Harry Potter and the Sorcerer’s Stone” with her friend the other day when one of the characters referred to Lord Voldemort as “You-Know-Who.”

Now I’m no Harry Potter fan, but I do know that Voldemort is the bad guy in the film, and no one dares speak his name out of fear of being heard and then getting snatched away. They also call him “He-Who-Must-Not-Be-Named.”

When she heard the movie characters mention “You-Know-Who,” Angie turned her face away from the screen and told her friend something that truly broke my heart.

“That’s what my Mommy calls my Daddy,” she said.

I felt a very deep sense of shock and shame.

Over the past three years, I have made it my practice to speak only positively about my ex-husband in Angie’s presence. When friends and family members have asked about the divorce or its aftermath in front of my daughter, I have responded elusively by spelling his name, speaking in code or answering them out of her earshot. And yes, I have referred to him as “You-Know-Who.”

Until that Harry Potter moment, I thought she was oblivious, but now I know she wasn’t. Turns out my tricks weren’t all that tricky.

Something similar happened when Angie was 3-years-old. Her babysitter canceled and I had no choice but to take her with me to counseling. I desperately needed my weekly therapy session, but I had no one to watch my daughter. So I decided to take her with me.

While I vented about the stress of single parenting, Angie played in the far corner of the therapist’s office with a plastic dollhouse that was kept on hand, presumably, for emergencies just like mine. Angie rearranged the tiny furniture, tucked the dolls into their beds and sang to herself as my session went on.

But about halfway through, she stood up, put her hands on her hips and looked very cross with me.

“Hey,” she said. “That’s my Daddy you’re talking about!”

After that, Angie spent my therapy appointments at her grandmother’s house.

As adults, we think we are being sneaky when we swear under our breath or spell our former spouses’ names. We assume that toys and movies are occupying their attention or they are uninterested in our adult conversations. But who do we think we are fooling?

Children are smarter than we think they are. They need to be; so much of their world is a mystery and they learn about their environment intuitively, filling in the gaps with information gleaned from adults who are whispering, averting their eyes or spelling out things they should just keep to themselves.

We assume they aren’t tuned in, but they are. Kids are selective listeners when it’s time to clean their rooms or brush their teeth, but when it comes to something we’d rather they didn’t hear, they are all ears.

Angie loves her father, and I am happy she loves him. I would never do anything to discourage that. When she asks about him, I say only positive things. What happened between him and me has nothing to do with her.

As she gets older, the questions will get more complicated, I’m sure. My policy has been to tell Angie as much truth as she can handle, with facts instead of opinions and as much neutrality as I can muster. That policy will never change. People who take the high road never run into regrets.

Angie is free to talk about her father any time she chooses. She chats with him on the phone and on Skype, and tells her friends all about how he fixes lighthouses and roots for the Red Sox. His name will never be taboo in our household.

But from now on, there will be no more spelling his name or covertly whispering to other grownups. No more “You-Know-Who.” Anything I have to say about Angie’s father will have to wait until she’s fast asleep, off in school, or far, far away at her grandmother’s house. If I don’t have something nice to say, then I won’t say anything at all.

Wendy Fontaine’s “Party of Two” column appears the first and third Sundays of the month. Her e-mail address is: party2fontaine@gmail.com. Follow Party of Two on Facebook and read her blog at PerseveringParents.com