Ok I figured it out !
To properly load a TerrainQuad from a j3o file created in the sdk you need to do something like that
Node loadedNode = (Node) assetManager.loadModel("newScene.j3o");
TerrainQuad terrain = (TerrainQuad) loadedNode.getChild("terrain-newScene");
//Where terrain-newScene is the name of you TerrainQuad child
PS : For both of us english isn’t our native langage that’s why we have some difficulties to communicate but it’s fine people always find a way to explain their thought.