One Park or Multi Park Tickets

We are considering going to US/IOA for two days during our upcoming WDW trip which will be 12/7 - 12/14. We have never been to US/IOA before. It will be my wife and I and our two kids ages 6 (girl) and 10 (boy). For two days, would you recommend to get a one park ticket or a multi park ticket? Are the parks such that we would probably want to spend a full day at each park and would probably not want to or have time to "hop" to the other park on the same day? We will not be staying at a Universal hotel and will not have the express pass. However, our son has autism, and based on my research, I expect we will be able to utilize their guest assistance system. Our daughter is just over 48" tall. Our son is around 55" tall. However, he does not like to ride most rollercoasters.

Also, do they have rider-swap similar to like WDW has, and, if so, how does it work?

I always suggest to spend at least one day in each park....especially if you have never been before. I would save the extra money and not get the park hoppers.

We definitely have not had time to hop! There is lots to do. I used to find IOA didn't fill a whole day, but now with Harry Potter it sure does. In addition to the rides there are lots of great shows...better than Disney (sorry) that will be great even for your young kids.

2nd that....especially the shows. Don't miss the Horror Make Up Show....it's so funny.