Last month, a few days before we finally put my mother into a home, I sat on the couch between her and my 6-year-old son, one asleep on each shoulder. Both had peed in their pants. As I sat there, sandwiched between the two of them, I realized they were the only people in the world who could do this and not bother me at all.

My 75-year-old mother has frontotemporal dementia. That’s not the kind where an elderly individual can’t remember what you just said but may recite for you a poem she learned in grade school. FTD has reduced a stunning intellect — my mother got a Ph.D. in English from Columbia University and worked as an English professor until she was 40, when she became a physician — to someone whose last vestige of recognizable cognition is to lean down to kiss the doll she carries around. People with FTD live an average of five to eight years after diagnosis, and my mother is nearing the end.

My son has a far less serious, though somewhat mortifying, condition. Officially, encopresis is a physical and psychological state in which a child who is often constipated begins to associate pooping with pain. Unofficially, encopresis means my 6-year-old still sometimes poops in his pants. And this pooping gives him the freedom, he seems to feel, to occasionally pee if he’s doing something really important.

These conditions mean that my father and I are in a similar situation. We have to be vigilant to get our charges to the bathroom in time. We can name to the hour when they have last gone. The difference between those we care for is that while my son throws around words like “parameters,” my mother will now put a Lego in her mouth to test if it’s food.

My mother’s FTD seems to date almost from my son’s birth, where she was present, silently monitoring the gynecologist like the working physician she was. Nine months later, while he was starting tummy time, she began to dress oddly and become unaccountably furious. By the time he was 2½, he chatted so deftly that people on the subway commented on his language skills. She was well into losing language, speaking what sounded like Finnish in her confident doctor’s voice.

There was a time in our lives when their interests, cognitive abilities and command of language all seemed to match exactly. When my son was 4, on a family vacation, I left out a handful of Legos on the kitchen island for him to play with. When I turned around, I found my mother next to him, snapping together blocks with the same careful concentration. From time to time, still a grandmother to the core, she looked over with pride.

That was two and a half years ago. And it was being sandwiched between their pee that made me finally insist it was time for a home for my mother.

My father had taken one of his first full-day breaks in years to see “The Marriage of Figaro” with his sister. I didn’t want to deliver my mother in a wet diaper. It would be nothing to tell my son to take off his clothes and get in the upstairs bath. But my mother is beginning to forget how to stand and walk. To get her upstairs to the bathroom, twice a day, my father has to perform a delicate choreography on their steep, curving staircase that is terrifying to watch. I didn’t think I could get her to go with me.

I tried to think if I could have her lie down on a towel and give her a sponge bath in the living room. But would she even lie down or let me undress her? It would be cold for her, and terrifying. What would I do with my son while it was happening? It would be pretty terrifying for him, too. I didn’t want one of his last memories of his grandmother to be watching me cleaning her stark naked in front of a wide-screen TV.

Changing my mother would be the equivalent of changing a 118-pound baby, and I didn’t even know where the diapers were.

The home where my mother now lives is only eight minutes away from my father’s house. The memory ward is actually quite a lovely place, like some mix of a fancy student center and a Hampton Inn. There are all-day activities that mimic those in my mother’s former Alzheimer’s day program: puzzles, music time, movies and performers. The daily schedule is posted on a colorful calendar at the front door, exactly as they used to publish the schedule at my son’s old nursery school.

My parents, who always drove their cars into the ground and didn’t seem to notice a hole in the kitchen wall for a decade, can afford it, thanks to their frugality.

While my father completed the paperwork with a representative from the home, I went shopping. I went to Target for the all-natural bath and hair products the director said the residents had to use, and bought a comforter and sheets from their “dorm room essentials” section. (My parents still use sheets and blankets from 1983. I was not sending her with those.) I ordered a bed from Ikea. I arranged movers for the things we were taking from my parents’ house.

It did not escape me, as those movers came, that I was doing exactly what I had done to prepare my son’s nursery before he was born. He had also had a room in my parents’ home. Now, we gave my mom the furniture from it: the Overstock recliner I’d nursed him in, the light that stood behind it, my old ‘80s bureau. There was no room for my mother’s bird’s-eye breakfront, her turn-of-the-century chairs.

After I put my mother’s clothes away and snapped the shower seat together, I covered the bureau’s top with cheerful pictures of my mother leaping across a green field and cavorting on the beach with her grandsons. They were there to remind the caretakers who she had been in the world, who loved her there.

My son’s light fixture, a propeller on which hang several colorful planes, was left behind in the nursery. My mother used to hold him up to the planes to touch them, to teach him his colors, to tell him about the world. I kept a picture of that — for him.

Lizzie Skurnick is the editor of “Pretty Bitches,” a forthcoming anthology.