Hi,
Since you're writing your own ControlBuilder, why not just call base.GetChildControlType(), where tagName is the tag's name, such as "asp:TextBox" and the attributes is a dictionary of the attributes on the tag. I do not recommend manually calling Type.GetType() because you won't be able to map all the tags to appropriate namespaces, etc. and you'll definitely fail if the tag is a UserControl.
Though, even aside from all this, why do you need to write your own ControlBuilder? ASP.net already knows how to add child controls to a parent control. As long as you have ParseChildren(false) on your control, the right thing should happen without the need for a ControlBuilder.
Thanks,
Eilon
Blog: http://weblogs.asp.net/LeftSlipper/