Creating a calculated table with a slicer

Base on what I am understanding you want to see everything from the main table and just be able to filter on the Child table month column.  see notes below or provide and example of the output you trying to accomplish.

1st make sure you have join the tables correctly.  ID to Tittle. 

2nd on you Matrix/Table if you bring anything from your child table, like the total amount, the slicer should automatically work.  If you are only bringing all the items from main table and don’t plan on bringing in anything from child table. Then you probably need to change your table join from many to one, to both, then your month slicer will work.