Reddit reviews Tails Are Not for Pulling (Ages 4-7) (Best Behavior Series)

Agree about not leaving her unsupervised with the cat, but I also wonder if she's trying to get your attention - especially with a new baby in the house, she may be feeling like she's been replaced and the only way to get your attention is by acting out (hurting the cat, hitting the baby doll, etc). Try spending more one-on-one time with her (tough with a new baby, I know!) and giving her lots of praise and positive attention for the things she does right.

You might also read her some books that emphasize how pets have feelings just like her, such as Tails are Not for Pulling or Animals Have Feelings Too. When you catch her about to hurt the cat, remind her how that makes him feel ("Kitty doesn't like it when you squeeze him, that makes him sad and hurt!") and give her an alternative way to interact ("Can you pet kitty gently instead? He really likes it when you do that!").