There are lots of reasons that toddlers play too rough with babies..
1. They lack impulse control
2. They are excited
3. They think babies can play the way they do
4. They feel jealous and can't understand how to express or handle that feeling
5. They haven't yet learned!!
They are not being violent. Yes, it is triggering and scary, but they need you to calmly teach them how to interact safely.