Go to the great grandfather's page. Click on the edit icon next to the child and next to the child's name click Remove or Replace to remove the parents.
Just quickly, in passing ...
IF, they are, a Couple; and, the Parents, of a Child; THEN ...
Go to the "Parent-Child" Relationship; &, either, "Remove" or "Replace" Parents.
Here is a "Knowledge Article" in 'FamilySearch':
How do I correct parent-child relationships in Family Tree?
I hope this helps.
IF, not; THEN, please advise ...