your can access your user control hidden filed value by following ways
var hdn = YourUserControl.FindControl("yourhiddencontrolid");
Or you can also create a properties in your user control which is returning your hidden filed value like this...
public string HiddenFiledValue {
get{
return myHiddenFiled.Value;
}
set{
myHiddenFiled.Value = value;
}
}
and use that property in your parent page like this...
yourusercontrol.HiddenFiledValue