K.K.
18 mo is young to "deal" with such situations. I know with my 16mo old and 3 yr old, I try to teach them in the moment, sometimes using the nuaghty kids at the park as an example. I have also said things to kids at the park that are not being nice to my children, especially if it is not safe. Parents that tell their children to push back are terrible examples to their children. Good luck! My own children try this behavior between the 2 of them. I try to intervene when injury will occur and talk them through it when it is really heated. My oldest tries really hard to give the younger one another toy so she can have the one she wants.