Does the oldest child of a cognitively impaired parent make medical decisions or a consensus of the children?

It depends. What does the medical power of attorney say as to who should be the agent? Before you say there is none, check at the nursing home or hospital where the parent resides. The parent may have been required to fill one out before entry. In the absence of a medical poa, the court must appoint a guardian. If the guardian is going to make an end of life decision, the guardian must be permanent, not temporary.