You have to skip <version> tag in child, but keep the <parent><version> ... </parent> tag.
http://maven.apache.org/guides/introduction/introduction-to-the-pom.html#Project_Inheritance
One factor to note is that these variables are processed after
inheritance as outlined above. This means that if a parent project
uses a variable, then its definition in the child, not the parent,
will be the one eventually used.