Hi,
first off, current built-in Panel control (<asp:panel runat="server" /> ) in ASP.NET already works this way.
However, to get same functionality for custom server control, there's two things to note:
1.
If you derive from System.Web.UI.WebControls.WebControl you'd need to specify the control with specific ParseChildren(false) and PersistChildren(true) attributes. ParseChildren(false) causes that page parser parses the content between control's tags as child controls and adds them to the Control collection of the control. This way control can have nested child controls in declarative model as well. PersistChildren(true) is for visual designer so that it persists stuff as child controls (inner content for the control).
2.
If you derive from System.Web.UI.Control, things are already working this way(content is mapped as child controls)
Thanks,
Teemu Keiski
Finland, EU