PSN - unhelpful and impossible

My children did a 14 day free trail of PS plus on their accounts behind my back. The 14 day trail has lapsed and they automatically deducted my bank account. As stated in the terms and conditions, but what does a 9 and 12 year old know about that? When I contacted them in regards to this they were not very helpful. First they said there is a 14 day cooling off period but that starts the day you sign up for the free trial and they would not refund me. I explained that it was my son who did it and I feel that there should be precaution taken on their systems so that children can not simply purchase what they like without parents consent. They now want me to call them with the following information: Online ID or Sign-In ID (e-mail address). • Details of the payment method in use on your account. • Serial number of the first console used to create or log into your account. • Serial number of the console currently used on your account. • Details of a recent transaction (including Order Number, content purchased, price and date of the transaction) made on your account.

So where do you find your 1st console serial number?

They simply are not interested in helping and is making it near impossible to get the refund.