Two times, but both are similar incidents:

The first was when my oldest was nearly 1. He was running a fever and we called the doctor to see what to do. The doctor suggested a lukewarm bath to bring his temperature down. This seemed to help and my wife went to get a towel. As she came back, my son looked up at her, but it was like he was looking through her. Then, his eyes rolled back in his head and he went limp.

I grabbed my son and shouted his name. There was no response and he was turning blue. My wife called 911 and her parents as I put him on the bed. The only thing that came to mind was "stop him from swallowing his tongue." (This isn't a thing I later found out, but I was panicking.) The emergency crews came as he started breathing again.

At the hospital, they said it was a febrile seizure and it can happen with infants/young kids. It's harmless most times, but very scary.

The second incident was with my younger son. He started running a fever and the doctor recommended a lukewarm bath. I think you can see where this is going and so did I at the time. I told my wife to get the towel beforehand. She also called her parents and they came over before we put him in the bath.

Sure enough, our younger son was fine until he stopped responding. Only he didn't turn blue. He turned grey. And he didn't start breathing again on his own. We called 911 and my mother-in-law did rescue breaths on him.

I was running from the front door, looking for the ambulance, to the bedroom - watching my mother-in-law trying to help my lifeless son breathe. My father-in-law told me that I didn't need to run back and forth. He said he'd look for the ambulance. I told him that I couldn't just watch my son lifeless like that. I needed to DO something. Even if it was completely useless, I needed to be doing something.

My younger son finally started breathing on his own and was fine. He went on to have many more febrile seizures until he (FINALLY) grew out of them. (He also had a tendency to fall and hit his head thanks to a hip issue. I swear that half of my grey hairs are thanks to him!)

I got the tiniest glimpse of what losing a child might feel like and it was scarier than I could handle. Even writing this out is raising my anxiety. I never want to feel that way ever again.