Living out in the SF Valley, where I get all sorts of wild critters, I doubt it was a possum.
My dogs have cornered several possums, and although possums look pretty nasty with those sharp pointy teeth, they generally really do just curl up and play dead when attacked.
SJ, I don't know where you live. Anywhere near a wildlife or mountainous area? If so, it might be a wildcat, which can be smaller in size.
In any case, you may want to also call Animal Control to report this. If they get enough calls which confirm yours, they'll come out to investigate. People don't like their small pets and kids getting "et" by the wildlife, ya know....
Good luck. And give your baby an extra dog biscuit -- he earned it guarding the house like that!!