You might want to try a fixed tensioner like the Rennen for a jumping/play bike. Never tried the fixed type myself. Tried the Surly, the Paul melvin and the old derailleur locked out with a couple inches of cable.
Old derailleur really was the most functional of the bunch. With the derailleur setup with a piece of cable you can adjust it with the barrel adjuster if it gets tweaked on a ride.
I did get the Surly to stop hopping around by removing the side plates and having it push up. That and cranking the spring tension. I found my frame would not allow push up mode without removing the fancy side plates, they hit the chainstay.
Fixed style tensioner:
http://www.rennendesigngroup.com/rollenlager.html