I went to a kids' Pokémon event. I expected child's play, but got a trading floor.

Edi, a 9-year-old Pokémon fan, told me a single card could be as valuable as a house. He was right. Lucia Vazquez for BI

Pokémon cards have become valuable assets on the playground and in cafeterias.

Preteen collectors aren't playing around: They view the cards as investments and a tool to build value.

I caught up with kids at a recent trading event. Here's what I learned.

My introduction to Pokémon was the cartoon, which premiered in the US when I was five. My brother watched and built a small collection of cards, only for them to be stolen, marking the end of his short-lived hobby.

I hadn't really thought much about the pocket-sized monsters since then, aside from when Pokémon Go became an inescapable phenomenon in 2016.

Kids take their binders everywhere — and not because they are interested in spontaneous games.…