Terrain tree from 2 crossed sprites/quads

Hi, I'm want to create terrain tree from 2 crossed sprites/quads.

But when I'm trying to use this object/prefab as Terrain tree, getting this message: The tree GameObject 1 couldn't be instanced because the prefab contains no valid mesh renderer. UnityEditor.HostView:OnGUI()

So 2 child GO have meshes, but Parent don't contain them? How can I combine child objects meshes to use complete/parent GO as Terrain tree?