Use Node.childNodes instead of Element.children.

jsdom defines 'children' on Node, but that's not in the DOM; the DOM
(and domino) specifies that only Element nodes have the 'children'
property.  Use 'childNodes' instead, which really is defined on Node.

Change-Id: Ie3dc5b5f6c9c8b695ab78c04a9db632bd1133947