Someone can play Splatoon alone or with other players (who they may or may not know personally). They can pay a monthly fee to play online with other players and to access voice chat (which is for players aged at least 13 who have the Nintendo Switch Online smartphone app).
On the Nintendo Switch console, a player can decide who can see their online status which allows friends to join them in online play. It is possible in some games to play privately with friends.
Splatoon is only available on Nintendo consoles, which have parental controls. Parents also have the option of using the parental controls app for Nintendo Switch called Nintendo Switch Parental Controls, which allows parents to monitor what their children are doing online and prevent them from sharing messages and images with other users. They can also create a Nintendo ‘Family Group’ to supervise their child’s Nintendo Account including preventing purchases.