We had traveled in BA flight and those guys have provided us with some crayons, a carry bag, a coloring/activity book for the kid, she being the toddler, I guess they have age-wise kiddy stuff, so it helps a lot, because it is something new, the kid had never used before, it was a gift to her, so she made it a point to use it and she had a cute little backpack to carry them all...
I did not personally carry any food, water or milk as she was 3-1/2 when she traveled and ate pretty much what we did, a bit less spicy. make sure to order Indian vegetarian meals or kid meals in such case. Got some fruity chews for the time of take offs and landing with the altitude drops and rises, told her to make sure she chews or gulps during that time and thankfully the kid did so.
walking around, playing with other kids, the rest room use, sleeping, eating, watching tv, playing with crayons pretty much made up for the in-air time and a big relief.
Carry a stroller which comes in very handy during the transit, waiting period and also to carry the hand bags, jackets, sweaters, carry-ons whatever if the kid just feels like walking or just carrying the kid around if she is sleepy than carry her physically.
Flash cards/activity books/empty book/pen/pencil/crayons
easy to carry flash cards and activity books keep them engaged.. or anything that keeps them engaged in general.
A couple of change of dressing options, just in case of delays in transit or things like that.
Fruits/nuts/snack bars if allowed during the transit if you feel the kid might be cranky due to hunger in between flights and depending on the season and climate on arrival at the place of destination, some cold or warm clothing, like when coming from India in November, it was reasonably okay there but back in US, it was cold, so some warm blanket/hoodie jacket, socks and shoes. Thick shoes are the best option to keep them warm and also run around, slippers with them slipping easily, I personally find are messy :(.
The kid had been wonderful like she always is (touch wood), so did not feel the strain of the journey as much.. thanks a lot buddies who shared a lot more info earlier, it helped!