Actually you arrive at Atlanta Airport from GIG at its South Terminal, where you need to go through passport check, retrieve checked bags, go through customs clearance and - if transferring to another flight - re-check-in bag (the tag should indicate layover and final destination Airport Codes). This will take about 30-45 mins. Delta should have a lounge facility in the secured area. When going through customs check, inquire shower options. You stay in the same terminal for your connecting Delta flight and you will arrive in New York's La Guardia Airport at the Marine Terminal A via Delta Shuttle - ground transportation counters are on th Arrivals level for information on New York & New Jersey travel.

In general you can take a collapsible stroller either (a) up to the entrance door of the plane, where the attendant will store it in the hull with return on arrival; or (b) bring it into the plane if it fits into the overhead bin or under your feet. Contact your airline.