Your bathroom stereo speakers could be wired to the front LR channel speaker outputs of your AVR. I guess that the program played to your bathroom speakers would be mostly stereo which anyhow appears only on the front LR channels only. If you plan to route 5.1 audio to your bathroom speakers then you should buy an AVR with a downmix facility so you can use this mode when playing your bathroom stereo speaker system.
Alternatively you could use a separate stereo amplifier to drive the bathroom speakers and wire the DVD player stereo outputs to this stereo amplifier.