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 s...