The teenager is yet to respond to his grandmother's plea.
Following Thorburn's tell-all, a spokesperson for the Stefanovic family told TheFix, 'We are disappointed Cass would claim such inflammatory suggestions that we have alienated Karl's children from our family,' the statement read.
'Nothing could be further from the truth. Karl regularly co parents his children and has his loving family around them.'
They added: 'We refuse to keep a running dialogue of the children. There are custody and financial matters before the Family Court and Karl has abided by every single one of them and we will not jeopardise it. Family is often compromised when parents go through a painful divorce and we do our best to continue our love and support for the children.'
Stefanovic and Thorburn announced their split in September 2016, after 21 years of marriage.
The Today presenter is now engaged to shoe designer Jasmine Yarbrough.
