The parent should, probably, only have pluginManagement, not plugin.
The pluginManagement section of the parent pom is used to share plugin versioning and base configuration across modules. This is in line with the dependency management section of the parent, which allows you to configure versions and exclusions across modules. For example, you could configure the site plugin in the parent to provide a standard look and feel across all modules, while the modules override the site plugin with module specific details.