Kids entertainment when flying will differ with the age and interests of the child.
Young babies can generally be kept amused with very little, provided they are not hungry or overtired.
Older children are generally happy to play the computer games which are available, listen to music, or watch a movie.
They can also be actively involved in the packing process before the journey and make their own decisions as to what they would like to take on board (within reason!)
Things such as portable games or a favourite book or magazine.
Toddlers, however, are a more challenging class to keep entertained.
I think it is fair to say that you will spend a certain amount of time walking up and down the aisles of the plane, as no toddler likes to stay still for too long.
Books with flaps provide some amusement, as do stickers and crayons (although the latter may need some monitoring). There are a vast array of cheap colouring, dot to dot and other activity books available. If you are lucky one might be issued as a matter of course by the airline staff.
Or you can print them in advance and stash a few pages in your carry on bag for emergencies.
A toy with gadgets will probably go down well, but of course space is always an issue. It might be a good idea to buy something new just for the flight so as to extend the novelty factor as long as possible, unless of course your child has an existing toy that they are happy to play with endlessly!
Our little one is always happy to play with a smart phone or tablet.
Either way, it is probably best to stagger which toys and activities you bring out to be looked at and played with rather than pulling them all out of your bag in one go, however great that temptation may be!
If you are lucky (particularly if you request a bulkhead seat) you may be placed with other families with young children, and if you are very lucky, the children may interact and amuse themselves to some extent.
Even if they don't, you might find some moral support among the other parents!