Rob,

It is correct that there is no corresponding function on the client to accomplish that.  You will have to iterate through the descendants and expand each one.  However, calling toggle() would be incorrect.  This switches the expanded status of a node.  So if a child of the node whose children you are expanding is already expanded, it will then be collapsed.  You'll want to call .set_expanded(true).  I have written a small recursive function that will expand or collapse the children of the given node.

function expandChildren(node, expanded) {
            node.set_expanded(expanded, true, true);
            var child = node.get_childNode(0);
            while (child != null) {
                expandChildren(child, expanded);
                child = child.get_nextNode();
            }
        }

regards,
Dave Young