Thank you for your query on Healthcare Magic.
The small pea size swelling at the base of skull may be:
1.Enlarged lypmhnode which is present in the sub occipital region.
2. Calcification of a small hematoma
occurred due to a trauma.
Nothing to worry as it is small and not associated with any other problems.
Watch for any increase in the size or any changes in the swelling.
During next well child visit you can show it to your doctor and proceed accordingly.
Hope I have answered your query.