On SpanishDict this sentence is given as an example of "mantener":
Manténle la cabeza al bebé con la mano bajo el cuello.
Support the baby's head with your hand under his neck.
What is the exact purpose of 'le' as a suffix on 'mantén' here? It must be a IO pronoun but what does it refer to? Is this a case of "dativo posesivo"? Would "Manténte" give the same translation or would that indicate that the verb is "mantenerse" and have a different meaning? Finally, what would be the negation of this statement?