why are they powerless they said if I don't respond or pay they will start formal debt collection

my wife walked out leaving me with the three children. they stayed with me a few months but I couldnt cope so my mum took them in and brouggt them up. . youngest is 23 now. . I have just receiced a letter from a debt agency asking for 1600 pound but my mum claimed for them .. apparently my ex wife claimed for them too at some point . I only ever heard from hmrc once many years ago and told them I hadnt claimed and where was the money going to. . they said devon.. I have never lived in devon but the ex does. where do I stand on this. . I was luving in scotland at the time.