Red Colour Rash over the body if present could be some allergy rashes which may be of sudden onset along with itching. or it could be viral exanthematous rash where the the onset of rashes is there along with fever or assosiated symptoms. and it could be some infection also which needs proper examination. Application of calamine lotion or Anti itch lotion like Ritch lotion can be done  for soothing along with antiallergic medications initially.

Good evening It will be really helpful if you can provide pictures for a more accurate assessment For diaper rashes you can use Siloderm mixi/Ezinapi cream.Apply generous amount with every diaper change

Age and photos would have been very helpful to guide you. Also fever and itching if there. Rash free cream can be applied but too difficult to say with conviction Coconut oil has home based care. Avoid dryness. Avoid diapers if being used.