Interface Document inherits interface Node and specifies Node.childNodes. It works fine, but what is children? There's no such attribute in the specifications.
They work differently. If we have <!doctype html> node, document.childNodes contains it, but document.children doesn't.
From Matthew Flaschen's answer:
Element.children returns only element children, while Node.childNodes returns all node children. Note that elements are nodes, so both are available on elements.
<!doctype html> is a node is a DOM tree, so it is listed in childNodes, but it's a DocumentType node, not an Element, so it doesn't appear in children.
The reason you didn't find children in the spec you were looking at is probably because it's relatively new - it was added in DOM4 (which became a W3C Recommendation only in 2015).
If you need to traverse the Elements in the DOM tree (ignoring other node types -- such as text nodes), you will find children and related APIs (first/lastElementChild, previous/nextElementSibling) more convenient, but since they're newer, the old browsers don't universally support these APIs, so you should consult the compatibility tables and/or use a polyfill.
