I'm not very clear on what you want, especially what you mean by a week to be selected.
However, it is possible to have a different value, and formula, for the parent / child.
First, you have to change the summary type to formula (which means you cannot have a LOOKUP in that formula).
Then you need to find a way to detect if you are on the parent or a child. The most commonly used method is a boolean LI you can call "low level" set to TRUE with none as summary type. You can then use this line item to control your formula:
IF low level THEN "this is the child" ELSE "this is the PARENT"
If you need a LOOKUP, you can have a specific line item with the value you want and reference it in the formula above.
I would do something like this, I believe the Final Filter line item at the end here is what you're looking for. What I did was use 'Ratio' to get the parent level of Week to have a '1' in it even if the Week was not selected. See below for the blueprint.
NOTE the applies to on 'Technical' is blank.
The Ratio on Level 1 is Technical / Technical. The Ratio on Level 2 is Level 1 / Technical.