Hey James,
(I’m so sorry I’m a little confused by your response to Steve)
are you using a 3d application to create this scene? if so there are good DOF options within the 3d applications themselves.
If that doesn’t work try rendering them in separate passes and applying DOF options in AE. You can try Steve’s suggestion, or what I sometimes do for a quick DOF, add the ‘Lens Blur’ to the background layer.