My Xh used to do that all the time
He would leave the kids in the car, go into a store, or more than one store. One day I found the car, with my eldest reading a book to her younger sister while the baby slept. I took all 3 kids into my car, and was talking and playing with them waiting for their father to come out of the Grocery store (all my eldest daughter knew was that she had read a couple books). 40 min later he came out of the video store at the edge of the parking lot with a bag of groceries, and a couple movies, he actually got in the car and pulled up to the parking lot exit before he realized he was missing something.
the worst part is he did not stop the habit, I ended up eventually being the kid driver most of the time since I could not trust him. it was not even the beginning of the end, it was a big part of the end.
just my 2c on this
Make this world a Better, and more beautiful place, that you have been in it
Good judgement comes from experience
but Experience comes from Bad Judgment