FKV

const flat = (arr, childrenKey) =>
  arr instanceof Array
    ? arr.flatMap(({ [childrenKey]: c, ...n }) => [ n, ...flat(c, childrenKey) ])
    : [];


const result = flat(items, 'children');

function flat(arr, childrenKey) {
  const result = [];

  for (const stack = [...arr].reverse(); stack.length;) {
    const { [childrenKey]: c, ...n } = stack.pop();

    if (Array.isArray(c) && c.length) {
      stack.push(...[...c].reverse());
    }

    result.push(n);
  }

  return result;
}

// или

function flat(arr, childrenKey) {
  const result = [];
  const stack = [];

  for (let i = 0; i < arr.length || stack.length; i++) {
    if (i === arr.length) {
      [ i, arr ] = stack.pop();
    } else {
      const { [childrenKey]: c, ...n } = arr[i];

      if (c?.constructor === Array && c.length) {
        stack.push([ i, arr ]);
        [ i, arr ] = [ -1, c ];
      }

      result.push(n);
    }
  }

  return result;
}

// или

function flat([...arr], childrenKey) {
  for (const [ i, { [childrenKey]: c, ...n } ] of arr.entries()) {
    arr.splice(i, 1, n, ...(c?.[Symbol.iterator] && typeof c !== 'string' ? c : []));
  }

  return arr;
}