Parental Controls on the Nintendo Switch are really easy to set up. A complete description is available here.
The first thing to do is to make sure that your Nintendo Family is properly set up as it will make the whole process much easier. It has also the advantages of sharing payment methods. Visit Nintendo Family to get started.
Once your family is properly set up, your family group should look like this:
Using the Nintendo Switch Parental Controls App (Apple and Android)
Download the app and follow the prompts to set it up.