Thank you for your query.
The dent you are feeling is called posterior fontanalle.
It is a normal gap that fills up at the age of 6 months.
If it sis too big or does not close after 10months consult your doctor because of the history of the earlier babies.
They might need a cranial ultrasound
Consult your doctor if the problem persists.
If you have any further queries you are free to consult me.
Hope this answers your query.
Wishing a speedy recovery.
Have a nice day.