In a lot of ways, this type of behaviour is the same as biting. So my advice would be the same. First, Observe - is there a time when this happens more? Is it a response to something or is it just for attention? Does something happen that causes the behaviour? I have found that writing this down helps identify patterns. If there is a pattern, you can use that to limit or eliminate the trigger. Next, Shadow. Be right there whenever the child is in close proximity to the other children. You can stop it from happening or at least react immediately. Third, Limit interaction and redirect. Obviously, you should say something but keep it short, like just "no" so that the attention can be focused on the child who is hurt. Lastly, Separate. When you are busy, (like making lunch,) keep that child away from the others with gates or in a high chair. Give them something to do "away" from the action until you can be right there again.

This type of behaviour is normal, albeit very frustrating. But they will, hopefully, grow out of it!