Is it permissible for parents to visit the grave of their son who was buried with a woman in a single grave, and is it true that they will not see him in Paradise if they visit him in this world?

Visiting the grave of a son is permissible for the parents, provided that the female visitor adheres to the Sharia (definition) guidelines, such as covering herself and avoiding mixing with men. There is nothing that prevents the parents from meeting their son in Paradise because of this visit. It has been reported that a miscarried fetus intercedes for its mother to enter Paradise if she seeks reward from Allah for its loss, and that a miscarried fetus will drag its mother by its umbilical cord into Paradise if she seeks reward from Allah for its loss. It is worth noting that burying more than one deceased person in a single grave is a matter of scholarly disagreement.