Obviously then, there is either a fault with the website, which sometimes allows renewals for U8's to be processed as FOC and other times not, or there are two different methods being used to renew.
My son is listed as a Friend & Family on my ticket,
I logged in with my client id, selected season ticket renewal options, both myself and son were listed as being able to renew, I selected two seats (as I moved seats this year) and it was all processed as one adult and one FOC U8.