Your question arguably doesn't have multiple answers. There's one answer: the set of children (discounting different orders of the set).
The question "Who is her child" has multiple answers because it asks you to deliver a single answer.
The question "Who is her child" has multiple answers because it asks you to deliver a single answer.
I don't think a person speaking plain English in most contexts should be seen as "correct" to answer the question with a non-list answer, even if the question is shaped to expect one, unless there's an established confidence that the shape of the question wasn't made in error.
If someone asked me in real life who "my child" is on stage, and I had multiple children on stage, I would first say that I had multiple children there, rather than choosing one from the set. It would be most helpful for an LLM in my position to do the same, rather than infer that [because Timmy is niam's child, niam's child ought to be Timmy when queried].