If maven gives this error, it means that the child module does not recognize the parent properly. I've seen this happen with wrong versions.
You think you are defining the right parent, but you are not. As @jordan suggested, by eliminating the child-parent relationship, you can see it works.
You cannot define the version of a module to be a variable you defined in the same pom, and maven will not want on this.
Although you think it's 2.5.0, it's not...
I suggest you review this relationship definition in your project.
I hope this helps.