Stretching element to contain all children

You can use the childrenRect property for this:

import QtQuick 2.0

Rectangle {
    width: 320
    height: 200

    Rectangle {
        color: "BurlyWood"
        anchors.centerIn: parent
        width: childrenRect.width + 20
        height: childrenRect.height + 20

        Text {
            id: hello
            x: 10
            y: 10
            text: "Hello"
        }

        Text {
            anchors.left: hello.right
            anchors.leftMargin: 10
            anchors.top: hello.top
            text: "World"
        }
    }
}

However, note that using childrenRect in combination with using anchors.centerIn: parent in one of the direct children yields a warning about a binding loop.

Setting the width and height manually works, but is a little ugly:

Rectangle {
    color: "gray"
    width: label.width+20
    height: label.height+20
    anchors.centerIn: parent

    Text {
        id: label
        anchors.centerIn: parent
        text: "Hello"
    }
}