How to setup transformation rule to map if an item is a child member of a (another) dimension parent

If you want to take the api route, I’d recommend you have your rule getting all children of that parent the first time the rule is executed and save results in a collection  (you can achieve this using globals and storing all children in hashset).

then you just have to check if member is contained in the hashset.

in this way you avoid multiple api calls (one for each line matching the rule criteria and not mapped yet)

HTH