My son is autistic and i intend on getting the disability pass.
I don't usually advise not going...but as a mom to a child with special needs I have to say this isn't likely to go as you expected. An autistic child, 2 younger (infant) siblings, on a first trip, in the crazy-crowds of Christmas -- too many wildcard variables involved. Consider a different, less crowded time. Christmas to New Years' is the absolute busiest time of year at WDW. Busy like elbow-to-elbow, nowhere to turn, no ability to get away from the crowds, noises, stimulation.
You can use
DAS. Just be aware that if the standby queue is 120 minutes, your return time will be in 110 minutes. You will need to find something else to do for all that time, and you can't get another DAS until you use or cancel the prior. Standby at that time of year can easily be 2+ hours for most attractions.
This trip is going to be a complete surprise to our 5yo.
Not recommended for an autistic child. Has he been to a major amusement park before? How does he handle crowds and noises and overstimulation? Rides themselves can be too much. Many people with children on the spectrum (or other needs) find it best to spend time watching
YouTube videos to help prepare the child.
Unfortunately it can't be any other time in December due to my 5yo being in school.
At best he's in kindergarten. It won't hurt his education to pull him for a week. Even a few days around Thanksgiving would be a better time (crowd-wise) than Christmas, and you'd still get the "Disney at Christmas" decorations and feel.
The atmosphere of how Disney does Christmas is magical and I want my son to experience thar before he gets too smart to figure everything out. He already knows characters are people in costumes
Kids love Disney well into elementary school years. It's magical at any age. He's not going to outgrow that for a few years yet. Knowing someone is inside a costume doesn't negatively impact the WDW experience.
I know this is a "dream" trip for you and your husband. But I would seriously consider waiting another year or so. Carve out time with your son 1:1 closer to home this fall and winter. Spend time preparing your son for WDW. Maybe try a slower time of year for a short trip, and then do the Christmas trip after you've found what works and -- more importantly -- what doesn't work for your family. I'd hate for your "dream" to turn into a nightmarish disaster that you regret.
Good luck. If you push forward, please ask lots of questions to get tips.