Dunno if you're still looking for a costume idea, but I had thought of this for myself when I was ttc (yes, I was thinking of costume possibilities for my then unborn child. hee hee!)
Anyway, back to the topic. I had envisioned my husband to be a sushi chef (in a kimono style top and easy black pants, fake knives, fake fish, and a headband) and I would be the waitress, with our child as a sushi handroll (here's a pic of a sushi handroll http://preview.tinyurl.com/6xutdg
). The waitress costume would be similar to the sushi chef, except that you'd be carrying the "handroll", and maybe a couple set of chopsticks in your pocket. Your 3 year old son could either be another chef or maybe a ninja (don't ask how a ninja fits in with a sushi restaurant theme, except that they're both japanese... ^_^)
This would be a great newborn costume since they like to be swaddled, and the sushi chef costume would be easy to nurse in (and easy on a postpartum body) since its a kimono.
Happy costume hunting!