Depending on age: Gator rider (sp?), or other tag along might be best.
All children, under local law age, must have safety approved helmet.
Lights, everyone must be seen. Cygolight+hotshot are great.
Hydration: water bottles of choice + spares on parent bike.
Replacement tubes (one per bike, per tire size) to keep rolling.
Pump. My Aldi, dual pump (for $7), is fine.
Basic adjustment tools and tire levers. I add a pair of cheap pliers.
Sunscreen, keep away from and don't rub in eyes.
Med kit in waterproof container. Mine is mostly <=4" bandages.
Warning device (I deal with cars, so use an Airzound horn)