Oh Callie, it sounds to me like that kid needs to learn some respect, manners, and social skills. She should have a clue about how to behave by now. Apparently not. One of the first things I was taught by my mom was you don't tease or mistreat animals, the lesson had to be learned early because my uncle had a dairy farm with dogs and cats running around, another uncle had a beautiful Irish Setter, and other relatives had house pets. How to properly treat pets was learned by age four and I've been an animal lover ever since. Your poor kitties don't understand that you're not punishing them... you're really protecting them from a 5 year old who doesn't know any better because she wasn't taught any better. These situations are even harder when the in-laws are involved and you want to keep the peace. This kid needs a sit down to be taught that YOUR house rules will be honored and Your pets WILL be treated gently and with kindness at ALL TIMES. I realize the spot you're in here, but what concerns me is abuse or mistreatment of animals by young kids may be a sign of more serious problems to come.
Congratulations to your husband on his graduation!!!!!!!!