static findChildrenNS(elem, ns, name) {
    const schemaNS = shaka.util.TXml.getKnownNameSpace(ns);
    const found = [];
    if (elem.children) {
      const tagName = schemaNS ? `${schemaNS}:${name}` : name;
      for (const child of elem.children) {
        if (child && child.tagName === tagName) {
          found.push(child);
        }
      }
    }
    return found;
  }
Finds namespace-qualified child XML elements.
@param {!shaka.extern.xml.Node} elem The parent XML element.
@param {string} ns The child XML element's namespace URI.
@param {string} name The child XML element's local name.
@return {!Array<!shaka.extern.xml.Node>} The child XML elements.