Thanks for posting your query
there are couple of differential diagnosis for the condition you have described.
The most common of these could be a ring worm
because of age group, site of the lesion.
The other possibilities are localized psoriasis
, or lichen planus
. these conditions can be differentiated by clinical examination.
Thirdly, as the child plays outside ir could be an insect bite reaction.
As treatment differs in each category it is better to consult your local dermatologist
or a family physician.
In case of doubt it is always try a course of anti fungals before other medications are applied
Have a good day