What will I tell my children about my in laws?

We are estranged from my in laws who are my children’s grandparents. My fil was sexually inappropriate with me for years, mostly when I was a teen and my mil allowed it all to go on. So we cut them out.

My mil still sends gifts for birthdays, Christmas etc and signs them as “grandma and grandpa”. Eventually when my kids get older, I’m sure this will confuse them. It may raise the question of who they are, why don’t we see them. And what am I really supposed to say?

should I just remove the tag that they signed? Or put a stop to the presents? And in general what reason do I give for them not knowing the other side of the family?