blah.js

function unwrapChildren(node, tag) {
  const treeAdapter = parse.treeAdapters.default;
  const nodeTagName = treeAdapter.getTagName(node);
  const newEl = treeAdapter.createElement(nodeTagName);
  const childNodes = treeAdapter.getChildNodes(node);

  childNodes.forEach((childNode) => {
    if (childNode.nodeName !== tag) {
      treeAdapter.appendChild(newEl, childNode);
    } else {
      const childContent = treeAdapter.getChildNodes(childNode);

      childContent.forEach((item) => {
        treeAdapter.appendChild(newEl, item);
      });
    }
  });

  return newEl;
}

Would it be simpler to make this more specific e.g. unwrap em?
And basically say if it's em, empty the parent and fill it with the em's children?

Or is there an edge case I am missing e.g:

<p class="standfirst">
  <em>
    some stuff that is italic <a href="asite.com">Somethinghere</a> and more italic
  </em>
  Back to plane old straight up text, maybe some <a href="link.com">links too</a>
</p>

Would it be simpler to make this more specific

Yeah – sounds good to me. I haven't managed to get this working in any way, yet though. Not sure what's wrong with it. Shout if you have any ideas.