Try using two FollowSortingMap components - one for each NPC - and making sure that Also affect children? is unchecked.
Then check Offset original Order? on the child NPC, and give the Sprite Renderer an order of -1. This will mean his order will always be one less than the parent.
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try looking there as a start
Then check Offset original Order? on the child NPC, and give the Sprite Renderer an order of -1. This will mean his order will always be one less than the parent.