Each of my kids carry their own. I help pack (usually telling them they can't possibly bring all that they are dumping on the tiny bag) but that's about it. In their backpacks I have:
* one change of clothes including socks (learned this the hard way when I was a kid and puked over myself on a long flight -- sigh). I usually find something thin and roll it up at the bottom where it's unlikely to be needed
* books -- I take them to the bookstore and let them pick out 1 book, then stash until the trip and they aren't allowed to look at it until we are on the plane. Just like my mom did with me.

* DVD player and kid headphones -- these are relatively cheap and serve to entertain well
* water bottle -- make sure to empty it before security and you can refill afterwards
* food/snacks -- I try to make sure I have filling and non-high sugar ones
* plain paper pads/activity books/coloring books/pencils/crayons/wiki stix
* snuggle toy, which can often be a blankie or pillow pet
* possible toy of choice, depends on how much room I have
* for the next trip they will also carry beach towels to use as a blanket and as a towel when we arrive.
In mine, I have:
* tablet
* sanitizer and wipes (those tray tables are not wiped in those quick turnarounds so clean those every time you put them down)
* meds, both for me and the kids (FYI for your motion sickness, I have the same thing -- take Bonine at least 1 hour before take-off. It doesn't make me super drowsy and helps a lot. I also carry ginger lemon chews and that helps too)
* change of clothes for me
* selection of DVDs for the movie players
* headphones for me
* snacks/food
* water bottle
* books
* purse contents
* cards/game
* tissues
* dry washcloth (this invariably comes in handy for cleaning or if someone isn't feeling well (wet it with cool water and put on the back of the neck))
* lots of different sized plastic bags -- something gets dirty or we can't find a trash, we have some way of keeping it separated from the rest of the goods
Depending on the length of the flight, I sometimes pack a toothbrush and toothpaste. Just depends.