Hi FamilyTraveller
Welcome to FT
We had similar seating queries travelling to YYZ in October - the advice I received from Customer Services at T5 was
- Check in at T-24 (you're then 'in' the system as travelling)
- Get to T5 early and explain your problems
- Hopefully they can resolve
We followed this and all was sorted very quickly - you ought to be able to do the same at ATL
Merry Christmas
MrB