Update childIdx to use a string rather than just an index

[ewu63]

Type of issue

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• New feature (non-breaking change which adds functionality)

Description

Currently, childIdx has to be an integer which refers to the index of the child FFDs (by the order in which they are created in the script). It should be updated to use a more descriptive string which is associated to each FFD. Maybe we should add a name field to each FFD or something?