Kiddies water bottle mold or bottle rusting

I purchased 2 stainless steel Barbie water bottles for my kids for school, as the previous plastic bottles was not hygienically clean. When the girls return from school, we fill up the bottle with water & rinse it off the next day. Our little girl fell ill recently & we tried to trace the origin, only to discover that the bottles are either rusting or has some horrible mold growing on the inside. (we unable to really determine, as it’s in the bottle & difficult to see). This is extremely dangerous & careless of Checkers to stock these bottles that has a risk