I'm not an expert, but being an English teacher, the use of 'they' suggests importance on 'they' and awareness that a 'they' exists.
In English, we either use the pronoun or an object acting as a pronoun at the start of the sentence to illustrate importance in context. If my car was stolen, I'd say 'my car was stolen' as my car is what is of importance.
If I say 'Jack stole my car', I say 'Jack' (pronoun) as that is completely relevant to context.
I'm not sure why this hasn't been picked up on more. I mean, the important object in question here is Maddie. Surely you'd say 'Maddie is missing' or 'Maddie has been kidnapped/taken'.
Saying 'They have taken her' implies knowledge of whom 'they' are. It makes no sense whatsoever.