Q. Since the Halacha dictates that youngsters, boys from twelve years on, and girls from eleven, if they make a neder or vow it takes effect, should they be also included in the above process of Hataras Nedarim?

A. Kovetz Halachos (5: 10 – p.55) mentions that the tradition is not to do so, since it may not be effective.
Although at that early age the neder takes effect, they may not be absolved yet from it until they reach the Bar or Bat Mitzva age (See Mishne Lemelech – Nezirus 2: 13. Halichos Shlomo 1: 38. Mishna Berura 571: 5).

Rabbi A. Bartfeld as revised by, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a