When my boys were in kindergarten, they were fully potty-trained of course. But as all kindergarten kids, they always had to have an extra set of clothing in case of accidents.
I think each of them had to change their clothing maybe once or twice from the time they were in kindergarten to maybe even grade 1.
It’s common – kids hold it in because they’re too busy, having too much fun, and then they wait until the very last minute to go pee and don’t make it in time.
It can be an embarrassing moment for these little ones! Especially if their friends happen to notice the accident.