I love my inlaws. I miss my mother in law nearly every day in some way. I hate when people can't get along with their in laws. They raised their children to be the adult we love. Seems sad we can't look past the "role" they play in our lives. If they were your neighbor, would you enjoy chatting with them over the fence? Would you be friendly with them? Why does it matter if they have the label of "in law", how does it make a difference?
Anyway, you have to decide what outcome you want.
Do you want to sit through a situation where your husband could absolutely go ballistic and rude and hateful at any moment or act like a hiney because you forced him to go?
Do you want to go to your family get together and take your kids and enjoy your day?
I would hate to force someone to go and then risk them letting out all their anger towards my family at any given provocation whether it as on purpose or some imagined slight. I wouldn't be able to enjoy the day at all.
If you can barter with him and work out a compromise that might make it a better day.