When considering clothing when flying, always take some changes of clothes as cabin luggage for any baby or toddler, as well as for yourself.
Make sure all your children wear comfortable, breathable clothes (which they can sleep in comfortably). Even if your child remains pristine, a change of clothes midway through the journey or at the final destination can sometimes do wonders if they are getting tired and tearful.
It can also be refreshing for the parents to put on a change of clothes! Consider wearing layers as, with many long haul flights, you may experience difference seasons (and therefore significant changes in temperature).
Also, sometimes the temperature in the cabin can vary, with some flights being warm and others being quite drafty. Particularly if you are travelling with a baby, take a little hat in case it is cool or draft in your part of the cabin.
One other suggestion – you might want to label your children's clothes. One fear of mine is to lose sight of my child in a foreign airport and have to explain to someone whose first language is not English what has happened and who my child is. I feel that having a name label in their clothes might just help, were that to happen. Carrying a recent photograph is also a good idea. Also consider purchasing some disposable ID bracelets.
Even if your toddler/young child is toilet trained, you might want to take a couple of disposable nappy pants, in case of emergency (eg in case of a tummy upset).