This is admittedly not at all clear in the April 15 draft of the spec, but
the children of an Attribute are the values (the String methods are
conveniences for the HTML world, in which attribute values can't contain
entity references). So, an attribute could have a single Text node as a
value (in the simple case), or a sub-tree of text nodes and whatever entity
references expand to.
This is being re-worked and clarified for the next draft.
Mike Champion