At first.
I was also a kid doing this, my cousins and I held ourselves underwater with the ladder rungs in a swimming pool.
At first, yeah a minute was tough. But then it rapidly increased. Unfortunately I don't remember where we topped out, but I think ~3 minutes.
We would also swim pool lengths underwater(but it was a relatively small pool at a condo building). I think I swam 9 once.
They'd let us stay out all night at that pool, it was great. Florida summers don't really get chilly.
Such contests are more dangerous than they first appear: Many kids will grasp the obvious trick of hyperventilating to improve their time, but that can lead to abrupt unconsciousness and drowning.
https://en.wikipedia.org/wiki/Shallow-water_blackout
I know that now, and I would prevent any kids from breath-holding contests.
But I didn't at 14 years old, nor did my mom or uncle, apparently.
I imagine a safe version could be made... that would suck all the direct-competition spontaneity out of it. Like "do it on land", or "only in standing-height water and take turns with someone timing and acting as a spotter."
Yes, although as another commenter pointed out, being underwater triggers a response in our body that lowers our oxygen consumption.
We did spot for each other, even then, but I don't ever remember anyone thinking they almost passed out or anything.
Funny, I did exactly the same things in my childhood, in my cousin's pool.
But it was on the other side of the pond!