First I have to say that I absolutely LOVE Disney, visiting is one of the things that I enjoy the most in my life.
Saying that, we went for New Years, about three years ago and it was awful, the amount of people there was incredible, the park was totally full by 10 am in the morning, doors closed not accepting more visitors... you would say, we are talking about Thanksgiving not New Year... but what I am going is this.....
after a little while I decided that I was going to enjoy my day there, and if I could make it to the rides fine and if I could not, then so be it, a lot of couples were getting frustrated and kind of fighting, when I observed that I said, I am here to be happy not to be upset... after that I could not be happier, there is always so much to see and to experience, rides or not rides, there are always rides that you will be able to see and if you know a couple of tips, like which rides you should get the "fast pass" and you plan ahead... it should be more than ok.
All the special Holidays Disney put on different events and shows, for me is always magical.
If you have in laws that are even willing to pay, WOW, that is awesome!, not that they need to do it, but the fact that they even offered, for me is like there is no wrong there. Aniversary or Thanksgiving the fact is that they planned this particular trip for a long time and you had agreed to it before and what they want the most is to spend that time with all of you.
If your only reason is the crowds, I am sure that if you decide to go you will love it anyway, crowds or not crowds, yes it is better without crowds but with kids in school that most likely will not be easy to happen.
Just do a little planning ahead and you will be more than fine.
I hope you decide to go and that you have a blast!