honestly i would be more concerned as to how the parents will feel about the children being on another level of the house then you. I know after having spoken to my insurance and cps, that is something were to haven and they were on a diffrent level, that you can be charged with willful neglect, and not be insured for accidents.
Not trying to scare you, but a very good friend of mine was changed when a child in care fell and hit her head, purely accidental, but because she wasn't in the room she had cps close her down, investigate and had the police involved. I would hate to see any one have to go through what she did.
I would say to have the kids upstairs with you when your upstairs, even if they help cook, or colour on the floor.