Thanks for your query.
It could be meningocele
Condition wherein duramater has protruded in the form of a swelling in between spine defects.
If the size increases whenever the child cries or with anyother straining activity then it is surely any one of the above two conditions.
You need to visit either a Neurosurgeon or a Spine surgeon
Hope I have answered your query.
I will be available for your follow up queries if any.
Wishing you good health.