There has been discussion of parents coaching kids by learning the questions, but it seems to me that subtle things (like encouraging a child or giving subtle hints) could play a bigger role. A tester might even test differently with a parent present (e.g., maybe being more patient and allowing more time than if the parent weren't in the room). I don't know if any of these things happen, but it does seem like there could be an effect. At the same time, some kids may need to have a parent (e.g., very young children too anxious to be left with a stranger). I think it must come down to a judgement call on the part of the tester.