That's why I highly doubt China would do anything - we only basically supply China's economy, with everything from jobs to steel. So I don't think China would get involved.
If I was Obama I would tell Putin to pull out of the Ukraine otherwise he will have another Afghanistan on his hands - just this time the United States will be eliminating his assets via carpet bombings.
Also, there is no way Russia could ever defend themselves against the US in aerial combat or naval combat, and Putin certainly wont launch nukes, if he threatened and was serious China would certainly get involved and would put Russia down themselves.
Besides, China and Russia were never buddies to begin with. The United States is certainly more of an asset to China than Russia ever will be.