I will soon be flying with Air Baltic from London to Tbilisi, transferring in Riga. Along with my family, my friends' child will be travelling with us. The child is 11 years old and will be staying with our family for the duration of the trip.
What kind of letter or anything else do I need to get from this child's parents? I assume I'd need to present something to the airline staff in London, possibly in Riga, likely in Tbilisi passport control; and the same on the way back - to alleviate any concerns about potential child abduction or trafficking. Obviously, this child will be travelling voluntarily with their parent's permission.