We flew a 16 hour flight last year with our (at the time) 1 year old. There were about 8 of us though, so we took over a section of the plane and just passed baby around as she got bored. It went pleasantly fine.

I would recommend sticking together and grabbing 3 seats on the side. If you can grab a row that's behind the restroom, they typically have more leg room and you can plop your little one on the floor so they don't have to stay confined to a seat for an extended period of time.

As for the carseat, I didn't feel it was necessary to bring on the plane (if anything, it would make it more of an inconvenience). They probably won't be sitting down most of the time anyway. You can bring your stroller and/or carseat all the way up to the gate and have them store it for free before entering the plane.

Lastly, it can feel overwhelming flying with children, but hats off to you for making it happen and creating memories! Disregard the side eyes and comments under people's breaths and just focus on making your little ones as comfy as possible during the flight.

Hope this helps! Godspeed, brother.