Thank you for your query
The lump may be a wide variety of things. But firstly it is difficult to comment with out having a look at it. And secondly most of the causes are not serious. Since your child is circumscribed, it is unlikely to be a collection of smegma
(which is the commonest cause of such small lumps on the penis. It can be a small cyst
. These are mostly harmless in nature and can be excised by a simple surgical procedure. But a visit to your pediatric surgeon is recommended.
God bless you