Do the parents bear sin for the death of their infant after taking him out of the incubator based on their insistence, and do they receive the reward for the death of an infant?

May Allah comfort you and compensate you with good. The Prophet, peace be upon him, promised a house in Paradise called "Bayt al-Hamd" (the House of Praise) to whoever loses a child and endures with patience and hope for reward from Allah.

The cause of death is to be determined by doctors. If the doctor permitted the infant to leave the incubator, then it appears that this was not the cause of death. However, if the doctors state that removing the infant from the incubator was the cause of death, then expiation (kaffarah) is obligatory upon the one who ordered it, and blood money (diyah) is obligatory upon his 'aqilah (male relatives on the father's side) to the infant's heirs.

If the infant was buried and the cause of death was not confirmed before burial, or the doctors did not state that his removal was the cause of death, then you are not required to investigate it, and the default is the exoneration of responsibility. If there is doubt about the cause of death, nothing is incumbent upon you, as expiation and blood money are only obligatory with certainty, not with mere conjecture.