Actually, the kid stuff is the origin of the word if I’m not mistaken. The idea is that it’s soft and it keeps moving. It’s opposed to the multiple choice dungeons where you can’t do what’s not in the list.
Now think Computer games. Old computer games where very rigid and you could either do what was in a list or do nothing at all. Then they created sandbox gameplay. They called it sandbox, because, like a sandbox, there was no multiple choice but it was more open ended. It’s no more “Someone gives you the mission you must do that”, it becomes “You are in the Kingdom of Whathaveyou, what are you doing ?”. It’s soft.
Well, that’s theory, because fact is in computer games, you’re still limited by the computer so it’s still limited by the computer so it’s not **really** open ended. And as I was saying, roleplayers are now using a word for something that we’ve been doing for ages without naming it. Basically, we just rediscovered the wheel. 🙂
Hope this clarifies it a bit. 🙂
Check this, maybe. 🙂 :
http://rpg.stackexchange.com/questions/662/what-is-sandbox-play