Stick to your guns. I like the "what you did with your children is your choice; this is my child" approach.
My ds is 10.5 mo and still hasn't had a Cheerio. I'm not comfortable with it. He hasn't had anything crunchy for that matter. I'd prefer he had more teeth/molars. That's my choice as his mom. That being said, I can see my mom experimenting if she wasn't told not to do something specifically.
I'm just trying to see why she would've given them...did she not have any other food or bm to give to your lo to where she was left to her own devices? She needs to respect your boundaries.