Brief Answer:
Most likely an enlarged lymph node

Detailed Answer:
Hello

Thanks for the query

I have gone through the details and do understand your concern. Although you have not mentioned the size of the swelling,as per your description it appears to be small.
Most common cause of XXXXXXX and painless neck swelling in this age is enlargement of lymph node.
Common causes of enlarged lymph node is infection in scalp,upper respiratory tract infection or physiological enlargement of lymph node
In most of the cases of physiological enlargement this swelling remains benign and decreases in size with age.
If the swelling is due to infection of surrounding area it decreases as the infection improves.
Sometimes the lymph node swelling can be due to infection of lymph node itself or other uncommon causes.
Here I won't get worried for the swelling but would like to observe it closely and if there is any rapid enlargement,pain,redness in the swelling or the number is increasing;I request you to consult a pediatrician for evaluation and management.

I hope this helps.
Please let me know if you have any more query.
Thanks and regards

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