I think I would explain to my grandchild that I had to tell his/her mum, it wasn’t negotiable.
Then leave it to them.
One of my daughters and I had a similar situation a few years ago, her daughter and her cousin, my grandchildren were found in their room watching something extremely inappropriate, they were about ten, we told them we were surprised and cross, and took away their laptops and phones.
We spent a long time wondering if we should tell my other daughter, they had already had a good telling off, but eventually we decided she had a right to know.
She didn’t seem particularly surprised or angry, which was definitely not the reaction I was expecting.
I think sometimes our generation are far more shocked than our children.