I saw this topic on reddit earlier today and some of the stories were pretty great. I don't really have anything from my childhood that comes to mind right now that applies to this. But I will keep thinking and contribute later on. Focus: What is a situation that you were in with your dad or if you're a dad, with your kids that you wanted the details withheld from mom?
I watched my dad slug a guy in the face when I was about 10 years old. We were at a local home improvement store on some random Saturday. We come out and get in the car, and out of nowhere a shopping car slams into the passenger side door. My dad jumped out and some random guy just pushed his car our direction without looking when he was done using. My dad got out and yelled at the guy, and the guy went fucking ape shit. He walked up to my dad screaming at him and pushed him. My dad just hit him without delay. The guy went down. He wasnt out, but certainly dazed and just got in his truck and drove away. He made me promise not to tell my mom and took me to McDonalds. Pretty sure he told her shortly after anyway.
My daughter didn't want to leave my MIL's house when I went to pick her up the other day, so I said she can ride in the front seat not in the back in her car seat as long as she didn't tell mommy. She said yes, and guess what the first thing she did when she got home?
My dad and his buddies had a hard rule on camping trips that "What Happens North of Ely Stays North of Ely." Thanks to them I learned what bourbon tastes like around 10 (thanks to a tiny sip of Knob Creek after announcing my curiosity). Lessons on how to properly light a cigar followed shortly afterwards. Mom never learned. We still turned out okay, I think.