Stuck...McCall's M5214

Halloween Sewing

So for my little man's Halloween costume I'm doing this costume:

McCall's M5214

 

Pretty much just exactly like the blue image on the left; which is actually 2 pieces. My concern is that it is not going to be warm enough for Halloween night here in Western Pennsylvania. 

**Remember last year??**

It was dark (duh), pretty chilly and raining. 

So it needs to be lined...and I'm feeling like layer of flannel just wont be warm enough.

AHHHH!!!!! I need ideas STAT!!

Pretty please :)