Hello,
i ran too in the same trap.
My scenario is Customer DocType with a Table-field to a ChildTable and the linked ChildTable contains also a Table-field to a ChildTable.
It seems to be really a fact, that nested Child-Tables are not working in ErpNEXT. This is a big problem, because now such a scenario must be solved in a not clean way. Nested Child-Tables are a common pattern in business-solutions.
Maybe it would be a good idea to make such a scenario possible.