When grandparents make a mistake

Yesterday we took my in-laws to breakfast for grandparents day. My FIL sat in the backseat with my son. He had a real struggle getting his seat belt buckled and unbuckled. Fast forward to today, I'm driving home from the grocery store, and I look in the mirror. I notice the middle seat belt, you know, the one that buckles the car seat in, is unbuckled and my son is just sitting there in his car seat being cute. I don't understand why a 61 year old man had such a struggle with a seat belt. Or why, knowing that your grandson is buckled right next to you, you wouldn't say 'hey I may have hit his buckle, you should check and make sure I didn't fuck it up' considering the worst case scenario in this situation is his grandson is killed in an accident, it's less embarrassing to admit you can't figure out a simple buckle. I'm enraged right now. I wanna throat punch someone. I probably won't say anything to him, so venting here is all I got. I would love some perspective on this one. Would you all let it go? Say something? Kill someone?