Ja'far was the cousin of the Prophet (PBUH) and he was martyred in the battle of Mu'tah. Although martyrdom is an honour but even then the bereaved family does feel the shock of the loss. For this reason, the Prophet (PBUH) permitted them to give vent to their grief for three days. It should not, however, be taken to mean that he allowed them to lament and wail, because that is prohibited. What he actually allowed them was the natural weeping which does occur when people come for condolence and speak of the deceased. Such expression of grief is permissible after a period of three days also.
Therefore, what the Prophet (PBUH) had advised them was not in the nature of unlawful but natural. The children of the deceased called themselves "chickens" as they were greatly enervated by the tragedy. This Hadith has been mentioned here to confirm the validity of shaving the hair of the head, especially of children, although keeping bobbed hair is more meritorious because the Prophet (PBUH) himself did so. Bobbed hair are cut short and allowed to hang loosely.