static findChildren(elem, name) {
    const found = [];
    if (!elem.children) {
      return [];
    }
    for (const child of elem.children) {
      if (child.tagName === name) {
        found.push(child);
      }
    }
    return found;
  }
Finds child XML elements.
@param {!shaka.extern.xml.Node} elem The parent XML element.
@param {string} name The child XML element's tag name.
@return {!Array<!shaka.extern.xml.Node>} The child XML elements.