Начните добавлять детей справа в строке

Просто установите направление текста.

    Row(
        textDirection: TextDirection.rtl,
        children: <Widget>[
          Container(
            width: 40,
            height: 40,
            color: Colors.blue,
          ),
          Container(
            width: 40,
            height: 40,
            color: Colors.red,
          ),
          Container(
            width: 40,
            height: 40,
            color: Colors.green,
          )
        ],
      )

Пример 2 с текстом

Row(
    mainAxisAlignment: MainAxisAlignment.spaceEvenly,
    textDirection: TextDirection.rtl,
    children: <Widget>[
      Text("one"),
      Text("two"),
      Text("three"),
    ],
  )