A parent with a coldsore kissed their kid and transferred the virus to their kid. That really sucks, but it’s not uncommon for parents to accidentally pass along HSV1 to kids by giving them a peck on the cheek when they’re unknowingly contagious.
Big bummer about getting HSV1 as a kid is that you can’t get antivirals for flair ups until you’re 12. So you have to be that crusty kid for a week or two at school… at least until high school.