IDK how things are now but I lived in buffalo up until 2 years ago and that border is reallllllly hit or miss. If you get a person in a bad mood you could get stopped. I'm not saying they won't let you back in but it could be a hassle. My friend got stopped (before needing a passport) because she had florida plates. No other reason than her plates were florida and we were going home to buffalo. My father got stopped once and his whole car inspected for no reason. So just be aware it could be easy, it could be a hassle...
personally I think if one person is required to have a passport to come back into the US everyone should have to have one. It shouldn't matter what line you come through and how miserable the agent is. They need consistency then there wouldn't be confusion.
Good luck and enjoy the trip!