var dataset = {
  tables: {
    customers:function(){
      var privateVar = 'foo';
      return {
        cols:[ /*here*/ ],
        rows:[ /*here*/ ],
        getPrivateVar: function()
        {
          return privateVar;
        }
      };
    }(),
    orders:{
      cols:[ /*here*/ ],
      rows:[ /*here*/ ]
    }
  },
  relations: [{
    parent:'customers',
    child:'orders',
    keyparent:'custid',
    keychild:'orderid',
    onetomany:true
  }]
};

alert( dataset.tables.customers.getPrivateVar() );