Included files are copied in a class, generated from your child template. I did something similar to what you are trying to acomplish.
Add a field to master's template, that contains Path to masters template. In Masters template just use it.
<#@ template debug="true" language="C#" hostSpecific="true" #>
<#@ output extension=".cs" #>
<#@ include file="..\\..\commonT4\master.ttinclude" #>
<#
PathToMaster = @"..\\..\commonT4\";