DLA overpayment recoverability

My client was in receipt of DLA at the middle rate of care component.  The original claim was made when he was 13 and was made by his foster mother.
Two months before his 16th birthday he went to live in the local authority’s residential children’s home.  His DLA care component should have stopped after a month,  but continued to be paid to his appointee until around the time of his 16th birthday, after which the payments went to a bank account in the claimant’s name and continued for another 4 months. 

The DWP have decided that the overpayment is recoverable from both the claimant and the appointee.  (I am only representing the claimant, not the appointee.)
I’ve not been provided by the DWP with copies of documents relating to either the change of bank account or the appointment made once the claimant became 16, so am disputing the overpayment on these grounds.

However,  I’d be interested to know if anyone has encountered a similar situation before.  At the date of change, the claimant was under 16 and none of the caselaw I can find on claimant/appointee overpayment cases involves a child.  As a child he was not obliged to report the change and could not reasonably be expected to do so, but can the overpayment be recovered from him because his foster parent failed to disclose?