for my dd first halloween I sorta splatter painted one of her little one piece suits and I dressed in "artsy" clothing with a big beret. when people asked what I was I said I was an artist and this (pointing to the little one nestled against me in her sling) was my work of art...easy and sweet.
another really easy but super great one is to go as a marionette. I found a neat old outfit (actually some traditional Austrian ensemble) and taped a staightened out coat hanger along the midddle of the back of the outfit up over my head. Then I put a cross shaped twig atop this and strung strings from the cross down to my wrists........a puppet.
I just found a really cute one we are thinking about for our family, same age children. Mom and Dad are farmers, mom wear's baby in front carrier decorated with the bottom half of an egg, baby in all yellow with a white felt piece cut to be the top half of the shell. DS/DD is a chicken or rooster.