You can say She rocked her baby to sleep if she was helping him fall asleep by rocking him. You can say She rocked the sleeping baby in her arms if he was already asleep when she rocked him.
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AnonymousShe rocked her baby (who was) asleep.-> She rocked her baby asleep.She rocked her baby asleep is correct, but it doesn't mean 'who was asleep'. It means 'so that he came to be asleep' / 'so that he fell asleep' / 'until he came to be asleep' / 'until he fell asleep'