My MIL 😤😤😤

Let me start off by saying, I have a good Mother-in-Law. She most certainly is not the worse, but I need a little advice as I am playing with fire.. Lets talk holidays and visiting. My son is 4 months old, this is his first time experiencing the holiday season! I am incredibly excited. BUT. Here is my problem. My husbands family wanted us to come visit them for Thanksgiving and Christmas, they are about 10 hours away from us. Well, that is a long trip with a little one, and wouldnt you know it, we got the child out of the cabbage patch that HATES the carseat. I mean he screams from the time you put him in the car until he gets out. So we told them we can only do one - either Thanksgiving or Christmas. They argued, specially my MIL, saying we have to come for both. finally they stopped.They want Christmas. Is it selfish to say no- we can come visit for Thanksgiving? My reasoning is, for, Christmas they are having 6 other people other than my husband, baby, ans I staying with them. That is a total of 11 people in the house 😰😰 We cannot afford a hotel since we are moving in the start of 2018. For thanksgiving, they are only having 2 people, so it is not as hectic. Christmas is an important holiday for me, and i honestly think I want to spend it with my son and my husband only. Is that understandable or should I just grow up and give them what they want?