Thanks for all the responses! I appreciate the ideas and experiences. I had already planned to tell them all it was an "oops"--no WAY they will ever know this baby was planned!! When I first started swaying, my MIL saw that I "liked" Fertility Friend on FB and made a really rude comment about it ("PLEASE tell me you aren't planning #4???????"), so at that time I knew we were in for it. I lied and said I was using FF to track cycles and prevent pregnancy, so at least I can say we were trying to prevent when we had our "oops".
I know my parents will soften to the idea, and they will love the new grandchild. I know their only resistance will to the idea will be out of concern for us financially. My MIL is another story. She STILL makes comments about #3, who is going to be 2 years old next week! It is really weird too, because she is from a family of 5, so you think she would understand large families. But they grew up kind of poor, and in turn she chose to only have one child herself, so I think she probably expected DH to have a smaller family too.
But Atomic, your point is so right on--we only have a limited amount of time to make our families! I honestly would have waited a few years for this baby if I could have until our finances were better, but I am 39, so I was out of time!