I watched this last week, In the programme a father reports his 5 year old son missing, hadn't seen him for 2 years and his solicitor advised him to call the police. It turns out the 'Mother' did not have a 5 year old child at all and had been using a friends child from a baby to build a relationship with the 'Dad' and to claim child maintenance.
The real parents knew nothing about it and the 'Mum' went to prison.