I am really upfront and honest with parents that there are other people interviewing for the same spots. It is common and parents understand. After the interview ask them to kindly let you know if they want the spot, either way and obviously if you are interviewing multiple families then I always acknowledge that I don't want them to feel rushed to make such an important decision but due to high demand the sooner they call the better. That way you are letting them know to have a sense of urgency surrounding their decision making but acknowledging at the same time, that ideally this would be a decision where as much time as needed should be taken to make such an important choice.

As for letting a parent down, no spot is a guarantee to begin with regardless of the competition. They aren't just interviewing you, you are interviewing them too so they should not just think that this is their decision alone. Let them know that obviously letting you know either way asap if they want the spot is the best way to go, but that you will honor all scheduled interviews prior to making a decision for yourself. Let them know that it is about finding a child best suited to your program and who will fit into the current dynamic of you group. This also reinforces to the prospective family that you give this a lot of attention and care about people in general to give everyone the opportunity and that you want the best for your other families and children too. It's all about good communication right from the very start and this type of communication could actually play a huge role on their decision to go with you as it gives them a great sense of who you are and your character.