custom ajax server control with nested tags in multiple layers

I am creating an Asp.Net Ajax server control and would like to know how to create a control with nested tags in multiple layers.

I could only find an example with one layer. http://code.tutsplus.com/tutorials/aspnet-ajax-server-controls-with-client-side-functionality--net-26166

I would like my control to look like this:

<FilteredCombobox runat="server" Id="fc-1"><SelectItems><SelectItem value="value1" text="text1"><Filters><Filter parentId="parentId-1" isNeutralParent="true" NeutralValue="neutral"><Condition parentValue="pv1"/><Condition parentValue="pv2"/></Filter><Filter parentId="parentId-2" isNeutralParent="false"><Condition parentValue="pv3"/></Filter></Filters><SelectItem value="value2" text="text2">
        ...</SelectItems></FilteredCombobox>

The control should render as a <select> where the <option> tags correspond to <selectItem> where the conditions in <Filters> are met.

Also is it posible to make an attribute mandatory?