Hi there Olivier.
Thank you for your detailed clinical description that gives one a graphic picture.
However, Dermatology is a visual line & as such, one would prefer a clear digital image of the lesion & you may upload it on this site using the upload data icon to the right of your screen.
As it is a child, it is best to visit a dermatologist
nearest you on the ground for best results.
From all accounts, it appears like an insect bite reaction & you may use a topical mometasone
combination cream or a plain hydrocortisone cream
that your local pharmacist may be able to help you out with.
Cheers & wish you & your child a safe & speedy outcome.
Dr Praveen Rodrigues MD