Why Did I Say That?

So here's the backstory: my youngest started kindergarten this year. For some reason, it hit me really hard seeing him transition to big-kid stuff, but also how quickly he's growing up compared to his older sister who already feels like a teenager sometimes. Anyway, we were at a family dinner and I was feeling particularly emotional about all of this. I made the mistake of saying something along the lines of 'I wish you could stay 5 forever, it went so fast.' To my shock, both kids got upset, thinking I didn't want them to grow up or age normally. My intentions were good, but they took it personally. Why do I always have to say exactly the wrong thing at the worst possible moment? Now I'm stuck apologizing and explaining myself for days. It was an awkward dinner indeed.