Unfortunately, if I want to change the position of child elements of the parent (ToolbarField is the 'parent' and all the buttons and such are 'children'), I need to identify the properties of the parent too, even though the size and position I want to leave to the default values. If you remove any parent tag, all the children will be ignored (and so left to their default values).

You can try to 'experiment' yourself by editing line 12 (in the SkinDefintion.xml file of Delving Hills (small) ):
x and y values are the position on the screen. The width of the bar is 1024 pixels. For any resolution with a bigger width than 1024 you should do a little math: (w - 1024) / 2 = x, where w = resolution width.
I'm running 1680x1050, so (1680 - 1024) / 2 = 328