The problem occurs for you when you go back to the page correct? If you would like this regione to maintain it's value you will need to store the value in a variable, then set the selected to that value with the dynamic properties of this select list. This will ensure that it retains the value that was selected. To make the child list update accordingly you would need to execute the function call that occurs in the onChange event in the regione select list. You would want execute this onLoad if there is a value for the regione select list.
As for getting the list to show all of the possible child values by default you would need to dynamically populate this child list with a recordset for all of these values. Then when the user makes a selection from the first list it should filter and limit the selections available on the second list. If I have misunderstood the situation and what you are asking for please let me know and describe a little more about the situation.