Be honest with her, but do it with a smile and a kind word. I used to have this "problem" a lot when we lived in an apartment complex where my ground floor apartment had a sliding patio door that opened into the yard.
I would say something like "Hi Sweetheart ! How are you today ? I am very busy today, so you may say hello to "Kitty", but then you must go back outside and play with your friends so I can get my work done. Thank you so much for understanding ! It's such a pretty day today, you really will like it better outside anyway, rather than in this old house."
Do it with a smile and stay right with her until you decide it is time to inch her toward the door.
On days when it's OK with you that she is in the kitchen, but she gets curious about the rest of the house, stop her when she starts that direction, and tell her you will be happy to show her the rest of the house, but she must have her grandfather's permission first. If after running home to ask, she comes back, take her on a tour of your home. At her age she is just curious anyway. End up back at the kitchen, and then kindly tell her that she must always, ALWAYS have your permission to go beyond your kitchen. She is not to be in the rest of the house without you.
Most 5-year-olds can understand and obey that. They just don't know the unwritten social rules yet. This is what they mean when they say "it takes a village" (LOL ... I hate that phrase, but it is true). We all participate in teaching children how to get along with others in this world. Occasionally her curiosity will get to her, but then you bring her kindly back to the kitchen and reiterate the rules. After all, 5-year-olds forget. LOL.
Sounds like you have a sweetheart of a visitor. I hope this develops into a wonderful little friendship. :o)