Mild concussion? Was there any fracture? Were there any symptoms? Ok if it is a minor trauma to head association with mild injury to nose then bleed can even occur some days after injury if the clot formed previously was dislodged due to breathing or movement of nares. A x ray and endoscopy of nose will get the diagnosis. Usually head trauma do not lead to nose bleed if it is minor trauma. ENT bleed occurs if severe head injuries. Sometimes CSF may be lost from nose in such cases. I do not thinkur child nose bleed is due to direct effect from head injury that is from edema of brain or hemorrhage in brain as it leads to many more symptoms like weakness seizures headaches vomitings altered brain functions. Do not panic and take child to your treating doctor,meanwhile pack the nostril only one which is bleeding with gauze piece like a stack of ironed clothes pressed together ,like a pile,like a treponemal shape,if it is heavy bleeding till u see doctor