Congrats! I don't know how you're going to keep it a secret - I would be bursting!
It would be great to get cast members into the "surprise." I think the easiest way would be to get a PS somewhere and call before hand to let them know you plan to surprise your family with the news and ask if they could help.
Just imagine with me - when you show up for your PS and give your name, the CM exclaims "oh, you must hot & tired! You really shouldn't be standing so much! Here - I insist you sit down while you wait!"

CM then flags down a server/busboy, "quick, get Mrs.___ something to drink!" Imagine the surprised looks on the faces of your family (as well as everyone else around during this big production!)
Fast forward to when you are seated - imagine the fussing & pampering by your server (and maybe some of the other servers as well?) "You should eat more, you need to keep your health up!" "You really should drink more milk," while bringing you a glass of it and refilling it throughout the meal. Etc, etc,... you get the idea. Have them hint but never actually say the news - none of "you're eating for two." By the end of the meal, your family should get the drift!
For some reason I think Prime Time 50"s at MGM would be perfect for this. Mom and Dad so concerned for your well-being, scolding you for not eating your vegetables and drinking your milk, etc. However, it might be more surprising, unexpected, funnier, if this happened some place more understated.
I don't even know if this can be pulled off. It's much easier to imagine than to do. But you can always call and find out if the restaurant is willing. Hey - they might really get into it!
No matter how you tell them, it'll still be the greatest news!!
Good luck! Let us know what happens!