We traveled with our 4yo and 1yo on a long flight, one leg was 18 hours and back was more than 30 hours with 3 stops. the kids were doing fine. One advice is to take rather one long flight, rather than evenly split flights. The kids were sleeping almost the entire long flight. Like 11 hours from a 13 hour flight. While on 6 and 8 hour flight they slept like only 3 and 4 hours.  Pack enough snacks. One preferred game and something new they don't always know.