A mom came to me concerned about her 3-year-old son. He seemed to enjoy hurting the baby and his older brother and sister, which made it even more troubling. The week before contacting me, the 3-year-old intentionally dropped the 10-month-old baby, which broke his leg.
Having a toddler that hurts the baby or a preschool child that hurts a younger (or older) sibling can be very troubling. BUT THERE IS A SOLUTION (and no one gets spanked). In the situation above, we had it all resolved in less than 2 weeks. The mom had been working on this problem for months. How did we manage to turn this around so quickly - behavior science. By understanding human behavior (and especially child behavior), I was able to help the mom change the way she reacted to the problem behavior (hurting siblings) and motivate the opposite behavior (making the baby laugh, doing nice things for siblings).
If you have a child hurting the baby, you need to watch, "Hurting Baby Brother: Sibling Rivalry with Aggression." The class is available in the Toddlers Classes. You should also watch "Parent Attention: An Awesome Influence on Child Behavior." These classes will teach you what to do.
The reason that a toddler hurts the baby is not because the toddler is an inherently bad child. There is some research that correlates such young child behavior with anti-social adolescent behavior. But it is not the child. It is the interaction between parent and child. You can make some simple changes and totally eliminate the problem and the possible long-term effects.
If you have a child that is "hurting the baby" then it is time to change things for the better in your home. Your whole family will be happy you did.