apart from reading the xsd for your particular webapp version, if you have eclipse,
you could create a dynamic web application and right click on the created web.xml
and click open with->xml editor and click on the design view tab.
Now by right clicking on the web-app you can add your child elements
For web app version 2.4 these seem to be the mandatory elements...
Although you got your answer I just wanted to show you a place where you can easily get the answer as well. There are nice Deployment Descriptor Diagrams included in the Servlet spec. Download for instance the Servlet 2.4 spec and got to SRV.13.4 Deployment Descriptor Diagram on page 135