Guest family is a form of inviting a kid to a family for the weekend or holidays. Any person, who had drawn up necessary papers and got a guest permission in the board of guardians and trusteeship, can invite a kid/kids from the orphanage to spend a weekend or holidays.
My first experience of communicating with the kids from an orphanage happened when I was 11 years old. That time I lived in a small town, where 6 kids from an orphanage studied in the same class with me. We made friends with three of them and every day we went to my place after school.
My personal experience shows, that there is nothing to be afraid of. I do not want to go into details and think if the kid takes the rules of the game, everybody writes here about them, or not. I just see that the kids are waiting for us and are happy to spend time with us, no matter what we are doing together.