My two sons (Jeremy - age 8, Brad - age 6) love everything about hockey cards. They can't wait to open packs to see what they get, then they have as much - if not more - fun looking at the cards and finding neat ways to sort them. Just the other day they decided to take their cards (probably a couple of thousand at this point) and figure out who was the heaviest player on each team. Before that it was the oldest, then the youngest. And they also watch enough hockey to have a pile for players that are not in the NHL, whether because they're still too young or have retired.
The expression "they can sit for hours . . . . " is pretty overused, but in this case it's really true in that sometimes they come home from school and go to their cards and then the next thing they know it's dinner time.
Dollar for dollar their cards give them more fun than pretty much anything else, especially since they don't care at all about jersey or autograph cards or what condition the cards are in - just like back when I was a kid, although in this case I'm pretty confident that what they're playing with won't turn out to be as valuable as the 70s cards that I wrecked.