I dont think babies under 6 weeks can fly, its quite dangerous, check with the airline, also remember the baby's eyes must be open for the passport photograph, they refused my pal's twins because their eyes were shut, we had to poke them & make them cry to get their eyes open, it wasnt nice and they screamed their heads off.
Another tip, when booking your flight ask for skycot for each child, most are free but some airlines charge a nominal �5, these cots hook on in front of you and you dont have to hold the baby for the length of the flight, although if there is turbulence you will have to take the baby out of the cot. If you have already booked ring the airline and explain you need a cot, also confirm at check in, believe me its worth the effort to get one, so many people have never heard of them, which surprises me.