The registration process for those runner under the age of 18 must be undertaken by the parent or guardian that are running with them. The best way to do this would be for the parent or guardian to register themselves first, and then go back and register the under aged runner. At the beginning of the second registration you'll be asked who you are registering for. Select the option 'someone else, below 18' and proceed from there. Now you'll be able to select an age under the age of 18, whereas this isn't possible if the under 18 participant tries to register themselves.