I made my sons, ages 2 and 4 each a robot costume out of cardboard boxes, duct-taped together and spray-painted silver. The arms, head, and the bottom were all cut out with an Exacto-blade. I drew gauges, and used pop bottle tops for on/off buttons. I had to cut out holes for the buttons and hot glue them in place, because I knew that other kids might be tempted to push them, and I didn't want them to fall off!
For their arms and legs, I didn't want to use dryer vents like I've seen so many times on homemade robot costumes, because I wanted them to be able to move around as freely as possible. So I used old shirts and pants and duct taped them as smoothly as possible. I had to cut down the back of their neck so they could fit their heads through! These worked great though, because they couldn't wear coats with these costumes, so they wore clothes under their costumes, and costumes worked great as a coat. It was very cold trick-or-treating! Their helmets were made of milk cartons, cut, and spray-painted silver with a plastic-bonded spray paint. I also used pipe cleaners, wrapped with duct tape to make them silver for antennas. I also used duct tape to make their straps. I just folded it in half and fed it through a small slit, then taped it in place. They never fell off, it worked great!
They wore black and grey gloves, and I cut strips of duct tape on each of their fingers and around their palms to make them blend in with the rest of their costumes! Their shoes, I put some of my husband's old socks over their shoes, then wrapped them in duct tape. I wanted to be sure they would have traction so I cut out the bottoms of them, and just did a small strap of duct tape under their shoes, to hold them in place. It worked great! I painted black dots all over the boxes, and used a silver marker to make X's in each of them to make them look like screws! And that's about it!
They LOVED their Homemade Bro-Bots Halloween Costumes and had a great time!