Just remove your numbers and have him register the Spark with his name to obtain a new number.If you are going out of the drone business and giving away your drone to your grandchild, is there away you can unregister your drone?
That would be for part 107 users who have to register each individual drone/UAV. The question appears to be out of context.From an FAA FAQ website...
If my registered UAS or drone is destroyed or is sold, lost, or transferred, what do I need to do?
You should cancel your registration through the FAA's online registration system.