I don’t want to spend Christmas with him

My child’s father and mother invited me to his grandparents house for Christmas. I don’t want to go though. His grandparents were very mean to me last year . We’ve forgotten about it though and moved passed it. I was just thinking about letting our son just go over there. I don’t want to go because I’d honestly feel uncomfortable especially since me and my child’s father aren’t even together. How do I say I don’t want to go without it seeming like I’m rude ? I’d rather be home alone