There was only one reason Kay could possibly think of for her mother to keep such a big thing from her..to keep her safe. From birth to aged 9 she showed no signs of being magical in any sense, the only thing she had her firey hair which was just like her mother's, except her mothers was shorter. She was close to her mother after all, and up until she disappeared/was killed, her family life was a happy one. She had a normal childhood just like anyone else. At the advice of staying away from old metals, she gave a single nod, "got it" she replied before she had to go and see to Serena. Chas shook his head at John's question, "none of us had anything to drink yet and I'm seeing it too" he said back to him. 

In the bathroom, she gingerly took Serena's shaking hands and held them to try and comfort her as she stammered how John couldn't know something. If John and Chas didn't follow her, she would have asked what he couldn't find out, but she wasn't going to do that with the two men around. "you're staying here tonight Serena, you nerd rest and I wouldn't be able to sleep and relax if you were out there" she told her,  letting Chas walk over and pick her up, helping her back to the couch, where he put her down. "why don't you let me see to her? She'll talk with me but not if you two are in earshot" Kay suggested, hoping that what she said was correct.