Q:

Create CAML SPQuery to order by ItemChildCount in descending order

How do I create a CAML query to display all items on the list order by ItemChildCount?

Somewhat like this:
 Select * 
 from table 
 order by itemChildCount descending 

A:

I believe the OrderBy Element is what you're looking for:
SPQuery query = new SPQuery();
query.Query = "<OrderBy><FieldRef Name='ItemChildCount' Ascending='FALSE' /></OrderBy>"; 

Adapted from the example here: SPQuery.ViewFields
Additionally, this is a useful CAML Query reference: Query Schema

