For the baby, I would try black or white leggings and a long sleeve onesie, then use fabric paint to paint the robot on. A Cheap pair of shoes could be used and spray painted to be the robot shoes. Similar thing for you, since you have to be 'bendy' to pick up the little one.
You could use card board from cereal boxes for your sons' robot costume. Two for each leg in front, two for each leg in back, painted whatever way you want, then use shoe lace to create a flexible link between the top (thigh) and bottom (shin) of the leg and do the same for the back of the leg. Another set of shoe laces could be used to keep the front and back of the leg together - so they aren't flapping all over the place.
That should get you started. Would love to see the finished products...