I definitely agree with the backpack--when my child was a toddler and I did an international flight by myself, I used an Ergo carrier to carry my toddler in the front and the backpack containing snacks, toys, extra change of clothes in a large ziploc bag (so you can put the dirties in the ziploc), diapers, baby meds (which was a bonus bc he had a fever on the return flight, so this was super handy), baby wipes and extra plastic bags, just in case. A duffle bag is too bulky and might throw you off balance. I also had a crossbody purse for other documents.

You might want to think about buying a child wrist holder, as I have seen some parents couple their child in busy airports so they don't get lost in the crowd. The other thing I recommend, if you can, is a child-friendly headset. The ones handed out aboard are not suited for children, if she were to watch the entertainment on the plane.

I also walked with my toddler up and down the aisles for a bit, which helps with the antsiness. Try to get sleep when they get sleepy, or at least be prepared for being sleepless.

It will be fine and you will do great!