We tried to schedule our flights around our son's nap times and bedtime IF it was possible.

Young children doesn't handle the 'heat' as well as older children and adults; therefore, it is important to not to overdress your child. We were on a Lufthansa flight from LAX to FRA and we were flying First Class which is the second level on the 747. My wife put on a thick long-sleeve shirt, long pants on our son who was 14-MO at the time. We boarded 45 minutes before take off, the AC wasn't running and hot air raises. He started to fuss and etc. over the heat...we changed him into an onesies and he was immediately fine.