It would be wise to get an X ray
done for the skull to rule out an underlying fracture.
A dent is caused by depressed skull fracture
and even if it is small, it needs to be treated.
At the same time, it would take time for the collection of blood to get resorbed. At the same time, do ensure there is no overlying cut or laceration as this could give rise to secondary infection.
All this requires monitoring, observation and care.
All the best.