Books where kids create their own world, like in Swallows and Amazons

I loved the S&A series partly because it involved kids that went off and stepped into a world where adults were distant, where the kids got to be the adults and become pirates, explorers, sailors. The same concept appears in the Paintball Wars Chronicles, where the kids go off into their own adult-free world and get to be the adults, in that case soldiers. Are there any other books or series like this?