You may do this also; add this code on child close button
<asp:ImageButton ID="btnClose" runat="server" ImageUrl="images/popup/close_bttn.png" OnClick="btnReload_Click" OnClientClick="$find('ChildPopupID').hide(); return false;" />
protected void btnReload_Click(object sender, EventArgs e)
{
//call the controls which you want to load
}
Also put all the controls should be inside update panel.
Hope this might solve your problem.