Suggest Remedy For Rashes In Children

my 1 year old grandaughter woke up this morning with red rash size of nickel with white ring around it. looked like a bug bite of some sort. it has gotten to be quarter size with white ring half dollar size around it now and she is saying boo boo hurts and will not let her mother touch it. i told my daughter i thought it was a poisionous spider bite of some kind and she should take her to patient first....what do you think?