I'd get some of that clear plastic for covering tables or whatever (they have it at Joann's), the thickest kind, or maybe enough for 2 layers... and then make it into a glass shape, and then for the milk either use some white fabric (felt maybe), or just paint the inside of the plastic.
Leave the bottom open for legs, cut 2 arm holes - maybe some sort of shoulder straps hidden inside the milk part.
Get a box and make it into a milk carton shape - typical 'box costume' base and paint it to look like a milk carton.
Or something similar but make a milk bottle kinda thing...
OR make just the milk - dress all white and make a sandwitch board type thing from fabric or cardboard and make it look like spilt milk.