In LLPSI cap. XX Orberg has Aemilia say:
O, miseros nautas, qui numquam domum revertentur! O, miseros liberos nautarum, qui post hanc tempestatem patres suos non videbunt!"
Clearly she's not addressing the nautae nor the liberi and so they could not be in vocative case.
But why accusative?
..and why especially when they are both the subject of the sentences (that end in passive verbs)?
Does the exclamation 'O!' take accusative the object which follows...? That would seem the most likely reason to me.