Thank you for your query
Such a mark can be anything
If your child has slept with pressure on that point, that may cause such a hardening. It may be some trivial injury
that might have missed your eyes. But you dont need to worry. As long is it is not red or warm or tender to touch', it should resolve on its own.
If it persists it would be wise to show to a pediatric surgeon.
Hope that helps
God bless you