I know a Sikh family disowned their son for cutting their hair and refused to acknowledge him as their son until he becomes Sabat Surat. What is Sikhi saying about those parents?

Respected Harmeet Singh Ji;

Thank you for asking this question. I am sure such things happen in any society. In other faith groups as well. So it is important to know the Sikhi view of this.

I am unable to find a direct quote in Sri Guru Granth Sahib addressing your question.
Please allow me to answer it based on my understanding of Sikhi.
First of all, we do not know the details of the circumstances at this home. Secondly, we are not the judge. So, passing any judgement is not our place.

In most cases the issue starts with the family environment. The communication between the parent and the Children is where it starts. Just because two individuals don’t agree on some one issue is not a reason to discard all else that they might agree on. Gurbani guides two parties to sit together and resolving the issues in an amicable manner. The resolution might be to separate.