What should a girl do who felt guilty for not giving a child sweets and yelling at her, to atone for her sin after it became impossible to find the child?

Upon realizing the error of terrifying a Muslim woman, one must repent, seek forgiveness, and feel remorse for what has passed. One should also increase in supererogatory acts of worship (nawafil), charity (sadaqat), and acts of righteousness. The Prophet, peace and blessings be upon him, forbade terrifying and harming a Muslim. Since you have not found the child you terrified to seek her forgiveness, you are only required to repent and seek forgiveness.