I would definately have a letter from Endo.  I presume you would be carrying a syringe in the emergency kit and it would be frightening if you got a syringe on board a plane these days without explanation.  I would also phone the airline prior to travel and explain the situation to them and they will advise on the appropriate documentation to have.   It would be a nuisance if you got to the airport and they wouldn’t allow the syringe on board and you were therefore unable to travel.   Wow, 8 hours on a plane with a 19 month old - GOOD LUCK, I think you may need it!!!