Answer:
Preguntan
Step-by-step explanation:
You use "preguntan" when there are more than two grandparents who are questioning their grandchildren.
you would use "preguntamos" if you and another person are questioning someone (example: " Yo y alexandra pregunatmos al maestro." )
You would use "pregunta" if only one grandparent was questioning. (example: " La abuela pregunta a su nieta.")
You would use "pregunto" if you are the one questioning the grandchild. (example: " Yo pregunto a mi nieta. " )