Hi , I am Dr.Kadhir,
The symptoms what have you mentioned looks like scarlet fever
This is most common in this age group (4 to 8 yrs), here the child has high fever
,bright red color in the creases of axilla & groin, sore throat
Treatment is doing a throat swab to confirm infection and treat with antibiotics as early as possible.
It could be a mild form of measles
but the rash occurs after 3 - 4 days of fever and also it starts from face (but rash usually occurs when there is fever), milder form of measles is quite common in previously vaccinated kids.
In both the conditions the child gets better soon on appropriate treatment.So it is very important that you show him immediately to a pediatrician
if he is sick.