The sunken spot you mention on her scalp
must be a small bump which feels soft. If she is normal now and able to do normal daily activities then i guess you can relax.The bump will settle down in few days.
However whenever a child incurs head injury
one must watch out for multiple vomits and abnormal behaviour in the next 24 hrs .If any of these happen then report to her doctor.At present if there is pain there you could give her a pain killer and put some ice packs on the area of hit.