SUBQUERY

Specify child relationship name and pass list of fields.

SELECT Id, Name, (
    SELECT Id, Name FROM Contacts
) FROM Account

public inherited sharing class AccountSelector {

    public static SOQL query {
        get {
            return SOQL.of(Account.SObjectType)
                .with(new List<SObjectField>{
                    Account.Id,
                    Account.Name
                });
        }
    }
}

public with sharing class MyController {

    public static List<Account> getAccountsWithContacts() {
        return AccountSelector.query
            .with(SOQL.SubQuery.of('Contacts')
                .with(new List<SObjectField>{
                    Contact.Id,
                    Contact.Name
                })
            ).toList();
    }
}