Isobel had kind of started looking at Cassie as a little sister. Since their first interaction, they seemed to hit it off and Mal wanted to continue to nurture that relationship. Plus, she wanted to offer her protection if and when any dangerous situations came up. After losing Evie, she didn't want to lose anyone else. She couldn't protect her best friend, but she could help Cassie.
Especially after that dinosaur drawing. There was no way in hell Mal was letting Cassie go exploring that park by herself.
"You want to check out some exhibits first or grab something to eat?" Mal asked. "There's pizza."