Thank you for your query,
usually develops after MMR vaccination
after 1 week to 10days of vaccination associated with mild decrease in appetite.
For rash you can apply the calamine
lotion externally and if there is itching you can give oral antihistamine in prescribed dosage.
Usually the appetite will come back in 1 to 2 weeks but if the child is still having the rash with decreased oral intake consult your Pediatrician
Hope I have answered your query.