It is definitely worth finding out what the procedure is in advance of the test. You should find the information in the admissions booklet or other paperwork you've received about the test. I would advise you to find out the beforehand as you don't want to be flailing around on the morning of the test.
I don't know the specific procedure for Kent but it is usually along the lines of; a. Ring the test centre to let them know, b. Obtain a medical certificate and send it in for a subsequent test date.
It is not advisable
to send a sick child to the test unless there are no alternative sittings. Sending your child to the test under these circumstances is considered a poor excuse at appeal. http://www.elevenplusexams.co.uk/appeal ... cation#b25
If they get worse during the test and are unable to complete it, they may not be given the opportunity to resit.