Family Working Tax Credits - Informing of changes

Hi,

I wonder if someone could help me?

My child has reached the age of 16 and is about to start work.

I informed Child Tax Credits as I know it will affect it - BUT - they said that I can't do anything till August?

I can't understand this and why they can't change the details immediately. I'm very concerned that I may get overpaid (I know you will say keep the money separately) BUT I'm more worried that it will be classed as fraud? How can I confirm that I have definately told them about the changes when the only way to contact is via telephone.

Thanks in advance for any help