Can the will executor agree to change the power of attorney to the children (who are named in the will as sole inheritance) to one of the children?

I think you are asking if an executor of an estate can resign and be replaced.  The answer is yes.  If the executor has been appointed by the court, he or she would need to  notify the court and the child would have to apply to the court to be appointed.  A power of attorney is a legal document that gives someone the legal authority to take certain actions on behalf of another person.  Estates do not have powers of attorney and  I do not think that the probate court in your state would recognize a power of attorney by an executor authorizing someone else to act as the executor.  But if all of the interested parties agree about who should be the new executor, it should be an easy process to get them appointed.