I will de-Lurk for a moment in the hopes of being at least mildly helpful. I have been twice in January, and was cold both times. Now, granted, one of those times was last year during the week that it sleeted, and let me say, Animal Kingdom in 28 degree rain is a whole new experience. Winter coats, hats, gloves, scarves, sweatshirt, and it still was too cold to be outside for more than one attraction at a time. But that was unusual, I realize.
My first WDW trip was in January of 2005 and it got cold then too. Of the 5 days of the trip, we got down to short-sleeves only one afternoon, and had on long pants every day. Generally a sweatshirt with a hat/gloves (easier to carry around in mid-afternoon than a jacket) was fine for most days, but early mornings and especially sitting still later in the evening for fireworks/parades (Fantasmic was especially chilly) could get cold.
On a February trip, it never got quite as cold, but long pants and long sleeves were still necessary, especially in the evenings. So, that being said, I would say that a warm hat and gloves are key items to bring. They are small to pack and to carry when not needed (whether in a bag or a pocket), they squish well, and it is surprising how warm a simple fleece hat can make you without needing a full layer of clothes. Plus, finding these items in WDW can be nearly impossible at times. Sweatshirts and blankets can be found last minute, but try to find a simple fleece/knit winter hat in the parks and good luck! (yes, I speak from experience)
Anyway, I am neither a spectrum mom nor a mom, but as an educator and graduate student, I strive to build my knowledge in as many helpful ways as possible and can't help but applaud your efforts (as fun as it may end up being for a little bonus vacation) in attempting to make things a little easier for another. Thank you.