It looks like your friend is talking about someone else's pets, not her own babies. According to the text, Lisa's cat (Tatze means "paw" in German, but in this context, it's likely a nickname for the cat) has had 3 kittens (Junge means "young ones" or "babies", but in this context, it refers to the kittens). So, there's no need to congratulate your friend on new babies, but you could certainly respond with something like "Aww, das ist süß!" (Aww, that's cute!) or "Ich verstehe, warum du jetzt auch eine Katze haben möchtest" (I understand why you want a cat now too)!