"Children's" means belonging or relating to children. For example 'children's clothes' or 'children's literature.' The form "childrens' " does not exist. The word 'children' is already plural, so you cannot add an 's' to it.

childrens не существует

Children is plural by itself ! But please see the following examples: 1- the student's library (it means the library of a student) 2- the students' library (it means the libraries of a student) I hope I could tell you the differnce between the " ....'s " and the " .....s' " Sincerely yours MehD ftim IRAN