If you can, take the late night flight. That way everyone will be asleep and hopefully, so would your toddler. Also be prepared for the worst. When my daughter was 2 yrs old, she developed a fever just as we were boarding a long flight. Lets just say there was much crying and lack of sleep.. Other than that, just do the usual.. bring their favorite toys, snacks, books. Also make sure they drink or chew on something during takeoff and landing when the air pressure changes.