A.W.
If you guys are into sports, you could do football (dad as a football player, mom as cheerleader and baby as a football or an umpire?) or baseball, hockey, etc.
I saw a cute group one time with the parents wearing orange safety vests and construction worker clothes and the kids dressed as orange traffic cones (I admit, it doesn't sound cute in this description but I thought it was hilarious).
Good luck!
Aimee