I've been geocaching with the kids for a couple of years now. It's a great way to get them outside and hiking or biking or whatever without having to hassle them to go just a little further. If they know there is a "treasure box" at the other end, they will just race along and spend all afternoon tromping around in the woods.
We've used bikes to get to many, but it's more of a rail-trail or nature preserve thing than a MTB type thing.
Oh, I'm dbwill while geocaching. Not sure where the whole "team" thing comes from, I've just used that one name.