Some BGOs like to do interviews at the candidate's home. As with Old Navy, I don't, but it's a personal preference.
I suggest the following: Have your DD answer the door. She can state that her parents are available when the BGO wants to meet them. Or, you can be in the room where she brings the BGO and meet him/her right off. If you have any questions about the process or about USNA, this is the time to ask. Just to ease social awkwardness, I suggest you come up with at least one question.
But it's not mandatory.
Also fully agree that, if there is a pause, you say something like, "Well, I'm sure you don't want to spend the entire night talking with us. This is about [DD]. We really appreciate the opportunity to meet with you, blah blah, blah." Well, don't say the last.
And take your leave. Meaning go to a place that is far away from where the interview is taking place so it doesn't appear you're eavesdropping.
The key is that the BGO believes this is your DD's idea, goal, desire, etc. and not yours. He/she may also be gauging if the parents are supportive. However, even if you think her attending USNA is the worst idea ever, that isn't necessarily a negative. I've had quite a few successful candidates over the years who've told me their parents were everything from apprehensive to outright disapproving. So, in that regard, be yourselves -- IOW, if you have fears, concerns, whatever, it's ok to express them w/o harming your DD's chances.
There is no need to provide food and doing so can be a bit awkward (BGO isn't into cookies but feels compelled to eat one just b/c you made them). It is nice to offer a glass of water or a soda, but that too isn't mandatory. Per my sticky above, your DD should dress as if she were going to the BGO's home or office (no bare feet, no gym clothes). You should wear whatever you would normally wear at that time of the evening, provided it isn't your PJs.
Dressing up too much IMHO suggests that you (not your DD) are the focus of the meeting/interview when you're not.