this seems to be case of viral rash
(exanthem) most like herpes simplex
infection. generally this appears in any body during stress period like fever, cold or any other illness. u need not worry much about this as is a benign illness if limited to one area and may resolve on its own with time. or best you can do is,you may consult your pediatrician
to suggest some medicine for local application, that may augment healing of lesions. maintain proper hand hygiene to prevent secondary infection and restrain from scratching at that site.
about cold and fever, it seems a seasonal flu that may respond to anti-pyretics like paracetamol
and some over the counter cold medicines.
watch for some danger signs like high grade fever, breathlessness, excessive irritability etc in which case you should contact your pediatrician immediately.
\the rashes of measles dont appear first at the site you mentioned and the character is different than your description, so it is a very remote possibility