How long does a head injury (harmatoma) take to heal? is it normal for my child to still be complaining of random pain in the injury area?

Depends: A hematoma can be small or large, disappear in a few days or last weeks & develop a calcium deposit over the bone where it formed. Headaches to the intensity of migraines can occur immediately or randomly up to 2 weeks after even simple head trauma. Without a better description of the size/location and age of the hematoma it is hard to speculate.