If there is a written will is place for 3 children and four grandchildren to inherit a property and one of the children has a child later do they get

Without actually seeing the will, it is difficult to speculate on what might happen.  If i read your question correctily, the person who wrote the will left her property to her 3 children and and 4 grandchildren.  This is an unusual way to bequeath an estate in my experience.  If the will has the correct language, then the additional grandchild will step in as an heir as well.  If it does not, then that additional grandchild is out of luck.   Typically in the situation you describe, a person would leave their estate to all of their children and if a child was dead and left children, that child's share would pass to his or her children.  If that's what the will says then in all likelihood the after born grandchild is probably an heir as well. You would be best advised to seek legal counsel.