HiddenValue in Usercontrol set by child User Control

Hi,
I have a webform containing a user control ucPageSuspect, which in turn contains another user control ucPerson.
In ucPageSuspect, I have a hiddenfield
<asp:HiddenField ID="HiddenFieldPersonId" runat="server" />
<usercontrols>
<asp:Button ID="ButtonAddSuspect" runat="server" Text="Add Suspect" OnClick="ButtonAddSuspect_OnClick" />

<uc1:ucPerson1 ID="ucPerson11" runat="server" />

<asp:Button ID="ButtonSave" runat="server" Text="Save" OnClick="ButtonSave_OnClick"/> 
ucPageSuspect
------------------
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
          &nb ...