Can my little sister take a stuffed animal with her on a plane?

My family and I are going on a plane to Colorado and I was wondering if she can just hold it with her through security or does it have to be on her carry-on? It's a stuffed Hello Kitty and kind of too big to put in the carry-on and she really just wants hold it with her. Is that going to be okay or will it be a problem? Thanks.