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 implements SOQL.Selector {

    public static SOQL query() {
        return SOQL.of(Account.SObjectType)
            .with(Account.Id, Account.Name);
    }
}

public with sharing class MyController {

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