Halloween costume for baby?

So while I'm sitting here trying to distract myself from waiting for labor, I thought I'd ask a fun question. Baby is due mid-September and has two big brothers so we will definitely be going trick or treating on Halloween this year.

Little girl will only be about a month and a half old but I'm trying to think of some cute and practical costumes for her (comfy, warm, uncomplicated--and hopefully cheap since she will only wear it for an hour or so??) anyone else dressing up their September baby?

I'd love to be creative enough to think of a theme for all three (2 boys, 5 and almost 2) but I'm pretty sure my 5 year old will want to be a ninja turtle!