Do you HAVE to have a passport to get back? In reality, no. But it'll sure make your life a hell of a lot easier. I know of some that have returned home without passports and I, personally, have never heard of a US citizen and resident being told they can't go home because they do not have one.
Realistically, they're not going to stop you from going home. However, they will need to verify you up and down to make sure you are who you say you are before they'll let you through. They'll do whatever they feel they need to do - can be a quick, easy process or can be a hard, lengthy one... all depending on who you get. The decision is ultimately theirs. It all depends on whether you're comfortable with that.
The smartest thing to do is to make sure you have the required documentation, like a passport or enhanced driver's license (for all parties). To not have it is to open yourself to the risk of having problems.
Keep in mind... if you are here in Canada and have an emergency (medical or otherwise) that requires you to go home RIGHT NOW, airlines will not let you on without one of the required documents.