And if there is no minimum age law for a child to be left alone, which there is not in Montana, then the mother would not be breaking the law. Would I leave a three-year-old alone at the mall? Of course not. But you have to separate stupidity from illegality.
Child endangerment law covers a VERY specific set of actions and behaviors, all of which are already against the law. There's not a catch-all "and anything else dumb" clause. But that's really a moot point anyway, because she didn't leave the three-year-old alone.
The three-year-old was with two twelve-year-olds, an eight-year-old and a seven-year-old. To put it in perspective, EVERY child in the group was old enough, by Disney's guidelines, to supervise that three-year-old through an entire day at any of the theme parks (but not the water parks, the minimum age there is ten).
The twelve-year-olds are the minimum age that the Red Cross, the Latchkey Kid Organization and numerous other groups consider old enough to babysit a child of any age, including infants. Nowhere in those guidelines is it specified that said babysitter is to remain home at all times and never take the kids to a park, an ice cream shop or even the mall.
Regardless, the twelve-year-olds made a less than ideal choice and left the three year old with the other kids for a few minutes (again, by the cosmetics counter in a store full of employees). That left a three-year-old, a seven-year-old and an eight-year-old together. In all but a handful of states, of which Montana is not one, the eight-year-old is old enough to be in charge of the three-year-old. In quite a few states, the three-year-old may LEGALLY be left entirely alone.
The mother broke exactly zero laws in her state. The twelve-year-olds had a momentary lapse in judgment and no one was hurt. That's a learning opportunity for the girls, not a case of child endangerment.
I just love the nanny state mentality. Let's don't let kids breathe without us until they're 18 (or 21, since they might accidentally taste a sip of beer) and then expect them to magically go off and be fully functioning adults.