Hello @kartik,
Use a JOIN in the DELETE statement.
DELETE p, pa
      FROM pets p
      JOIN pets_activities pa ON pa.id = p.pet_id
     WHERE p.order > :order
       AND p.pet_id = :pet_id
Alternatively you can use
DELETE pa
      FROM pets_activities pa
      JOIN pets p ON pa.id = p.pet_id
 WHERE p.order > :order
   AND p.pet_id = :pet_id
to delete only from pets_activities
Hope it helps!!
Thank You!