Headphones - you'll get free ones to use but they are usually rubbish and uncomfortable.
Do your older children have tablets, like iPads? Fully charged iPads, with comfortable headphones and a charging cable. Most long haul planes have usb sockets these days. Things to watch or listen to which have been downloaded. Same with games, etc - they need to be downloaded and not need Wi-Fi to work.
There will be tv, films, music and some basic games in the inflight entertainment, you can normally check what will be on by logging into the flight company website.
Comfortable clothes. I find planes tend to be warmer rather than colder these days. So maybe layers. You'll be given a thin blanket and small pillow. Thin pair of socks.
Bottle of water each - buy when through or take an empty water bottle and fill up once through security, etc. Most airports have drinking water stations.
Smallest one - some small toys, sticker books, magnetic games, etc. Ideally don't bring all out at once - space them out a bit. Magazines often go down well. Most airlines will give the youngest some form of 'children's entertainment bag' with some basic things in - not much but it often feels excited to get it when you're little.
If your children are a bit fussy with food - some simply, easy to eat, non messy snacks.