Madriver,
Don't beat yourself up to much. As a youth director I deal with this everyday.
Have you tried some indirect soft skills? If the conversation comes up again, you can say things like, "Yeah, Garfield...that was a very challenging and long hike for me." Or 'Wow, Garfield, what an interesting choice for your first backpacking trip with your kids."
Been my experience that indirect questions/statements help the person (parent) think about the issue at hand and don't lead them down the path of "I'm the parent, They are my kids. I'll raise them the way I want." Once any parent is in that mode, its hard to have constructive dialoge.
Like I posted earlier, I have done Garfield w/kids before. They are long days. You know this parent more than I so I tend to think, by your posts he may be a bit overwhelmed and not even know it. (Jeez...that make sense?).
Like to know how it turns out as well.