Individual powdered drink mix – Such as Propel or Crystal Light. It saves money since we don’t have to buy as many beverages on the go, it’s green because we recycle bottles, and it’s healthy because my kids are more likely to stay hydrated than if all they have to drink is plain water. It was especially useful in Europe where the selection of soft drinks was limited, often all the only options were Orangina or regular Coke.
Nightlight – I take one traveling with or without the kids, because hotel rooms and cruise staterooms often have blackout curtains. It’s easier to find the bathroom in the middle of the night, and prevents tripping over or stubbing toes on a suitcase in a pitch black room. They’re available at dollar stores so if I leave it behind accidentally, no big deal.
Power strip – A family of four often carries more electronic gadgets that there are wall outlets available, so a multi-outlet strip helps prevent arguments. I bought a dual voltage power strip from a travel shop for our recent trip to France, and it had a USB port as well. It allowed us to could get by with fewer adapters, too.
Extra ear buds and a splitter – I save all the ones we pick up while traveling, and keep them with our travel gear. It’s nice to have backups in case one breaks, or is forgotten. The splitter allows two kids to share one iPhone to watch a movie or listen to music.
Collapsible insulated tote – I have a fairly large one that is light and packs almost flat. It does not have thick insulation, but works well if items are already cold. It’s convenient & a cost savings to purchase drinks and a picnic lunch while traveling.
Swim rings, floats, and beach balls – I buy cheap ones for less than $2, and leave them in the package so they are flat and compact. Then we don’t have to buy or rent while on vacation, and each kid has their own (again, avoid arguments). When heading home, we can leave them behind or give them away. Though my ever-thrifty husband has been known to deflate & pack them down to almost the original size so they fit back in the suitcase. 
I'd say multi-port chargers are indispensable for all my travels – with or without my family. A few odd items can also be found if you rummage through our bags such as nail cutters – old and rusted you'd be embarrassed to let anyone see you use it so you never really do use it while travelling.
What a great, practical list. I really like the idea of bringing small packets of drink mix.