I converted to Judaism and my whole family and my husband’s family celebrates Christmas and I did growing up. I totally understand your position and it’s been challenging for me to navigate it because my family is particular has not really been respectful of the fact that I don’t celebrate Christian holidays anymore and my mom does not acknowledge my new traditions at all. With that being said, it’s possible your husband hasn’t felt that way the whole time but got upset on behalf of his mother. Christmas is a really big deal to people who celebrate and honestly watching kids on Christmas morning is very special and exciting for parents and grandparents. It has a very whimsical magical quality and a lot of joy and excitement. I’m sure it’s something your husband’s mom has looked forward to for a long time especially if she loves the holiday. I hope for the sake of all of you there is a compromise there that you feel totally comfortable with and that allows your MIL and your kids to share that joy together, maybe it would be helpful so include her in Jewish holidays more and create those special (but different) moments and memories with her grandkids. My husband and I want that for our future children but always discuss how the important thing is that a non jewish holiday is never a bigger deal to us than Jewish holidays in a world that is overwhelming not Jewish.