Have you ever happened to take care of other children? Did you treat the kids like you treat your own kids, or did you treat the kids like their parents treat them?
When I am taking care of other kids, I am the adult and they are kids. By virtue of taking responsibility for their care, I have authority, and they go by my rules.
It makes sense to treat them as you would treat your own. I am not overly strict to my kids but I draw the line if things start to get out of hand. I have acted the same with kids who were not my own as well. You are doing it for their own good.
That is exactly how I feel, even if I am watching the part time or not. We had to take care of our cousin for a year before when their parents were both recovering from a nasty car wreck. If you come into my home, you respect and follow my rules.
I am similar to the others, I would treat the children the way I treat my own children. I wouldn't want the children or my own children to think that I was being unfair in treating them.