Get(ModifiedFields) return modified fields for the main table of the current layout.
But nothing for tables in the same layout but with a relationship to the main table.
A calculation field in the child table can be set to Get(ModifiedFields). You must turn on "Do not store calculation results".
You could maybe use script triggers to check for changes that have been changed in related tables
Look's great, but...
What about the impact on the overall speed for this?
Sorting, displaying, ...
Because it's unstored? Minimal. It will only be evaluated when you refer to it, and it wouldn't make sense to sort by this field... When you do evaluate it, it's just like referring to the function itself which you are already doing in the parent table (ie: it's not stored there either).
Retrieving data ...