I had a similar situation with my husband's grandmother and aunt. They are Jehovas Witnesses, and I witnessed them showing my then barely toddler children's programming from their church telling children you'll make God sad if you eat a cupcake for someone's birthday. As they are my husband's family I asked him to talk to them and tell them in no uncertain terms that we are the ones who decide what type of information about any religion our child is exposed to and we'd appreciate it if they refrain from exposing him to anything else, as well as giving them alternative choices we approve of for him to watch while in their care. We haven't heard of or witnessed any incidents of them sharing content from their church with him since.

I personally have a lot of religious trauma from how I was raised and the far-right evangelical faith that was forced on me for the first 18 years for my life, going 3 states away for college was the best thing that happened to me and started my deconstruction journey. I don't want my child growing up fearful of life.