In a situation like that, I guess I look at it from the little girls' perspective. She is probably lonely, missing her mama and sees you as someone who is kind and loving. As a mother, I find that I tend to take responsibility for other peoples kids when necessary.
Case in point, most of my kids were grown. We have a backyard neighbor who has two little girls. At the time that this was going on, my son was maybe 10 and the girls were 9 and 7. They would come over randomly and ask if my son could play. I would invite them in and ask them if their mother knew where they were. It tickled me so one time when one of the girls said "our mom told us we could play for an hour"...and sent them over. I was fine with that...their mama needed a break and she felt they were safe with me.
Now, that being said, if I had chores to do, or errands to run, I would tell them now was not a good time. But if I was simply going to be home, I would invite the girls in and keep them for awhile.
It does indeed, take a village to raise a child...I am so thankful I get to be part of that village!