Most kids are used to a schedule, and they can get grumpy when snack time isn't exactly the same time every day. If they can't run around like they can at home, they might feel a bit trapped. So when planning a flight, you first want to think about your child's usual habits. If you're able to plan around that schedule, your flight experience will seem to fit more easily into an everyday routine.
Now, this isn't always possible, so the next best option would be to book a morning flight. These earlier flights are less likely to be delayed, and you're more likely to have better options if your flight is delayed. As long as your kids get a good night's sleep, morning is a great time to fly, because they'll be less tired and in a better mood.
There are some instances where you'll have to book a later flight. Still, keep in mind that there are pluses to evening flights. They can save you from some of the running around that might happen before early departures. You'll also have the whole day to get ready and plenty of time to arrive early at the airport. Remember, though, that late flights might leave you with a very tired kid who won't be able to sleep on the plane. And that can lead to crankiness, which can sometimes lead to crying and tantrums. This lack of sleep could affect the next day, too, so plan your travel itinerary accordingly. On these later flights, try to bring some calming music or a favorite video that bring sweet dreams to your child.