This won't help in this situation but for others do not be afraid to crack down on the kids when they are very very little for doing behaviours that seem bent on annoying others as it escalates when they get older and can become a major issue such as described here. Too often we let things go because they are little without realizing that what that 18 month old is not his way of trying to play with the other kids but is him being wilful and destructive on purpose. The sooner rules are put in place to limit the behaviours the more likely it will be over before the child gets older or at least you will have a system in place. One of the reasons we have more problems with children we get later in life than when we start them as babies.
Bullying is very much in the news these days and some studies will show that when they go back in a child's history that there were signs when they were much younger that they did not have empathy - the ability to understand how others felt or self-control - as in was too impulsive acting first and thinking later. Two skills we need to work on with toddlers so they are innate abilities as they get older.