If you choose to restrict gameplay on the console itself, you can set the age limits of different features, such as the games and whether the user can post screengrabs taken on the Switch on social networks. Communication with other players can also be restricted.
It is possible to link your Nintendo account with a child’s Nintendo Switch profile, so they can purchase games from the online Nintendo eShop. If you have that option installed, you can also restrict those. The instructions are on the Switch in that case.