There are couple of mistakes in your code, first of all passing arrays to FindAllChildren is invalid parameter, pass it string values.
Further object.Items is not an array. you can access Items properties by their names for example object.Items.IsReadOnly or object.Items.Count etc.
function GetContextMenuStrip()
{
var propertyNames = "FullName";
var propertyValues = "*ContextMenuStrip*";
var objects = Sys.Process("XXXXXXXX").FindAllChildren(propertyNames, propertyValues, 100, true); //(WHERE XXXXXX is the name of the process of my application)
objects = (new VBArray(objects)).toArray();
var object = objects[0]; //(IT WORKS UP TP THIS LINE)
ShowMessage (object.Items.Count);
//var array = (new VBArray(object.items())).toArray(); (IN THIS LINE FAILS)
// var a = array[0];
}