I have "Another One Bites The Dust" going through my head right now - not to sound trite about your situation. It's just that in the short time I've been reading and posting on this forum, these stories are an everyday occurrence. You'll easily find like minds here! And I am sorry about your situation, for you and your child (I have two daughters) and also for your wife, who now must face a painful reality. Just sucks for everyone involved and it really doesn't seem fair in any way.
Read, go to Al-Anon, and keep your focus on your child. Jayscott is right - your wife is an adult and can take care of herself (if you get out of the way and let her) but your child needs you and is completely dependent on you.
Take good care!
T