Mostly off the top of my head:

    document.getElementById('some-form').getElementsByTagName('input').forEach(i => i.classList.add('error'))

It is a little more verbose but it isn't that much more cumbersome today.

That's also doing things "the right way" and not do-all selectors, but that option exists now, too:

    document.querySelectorAll('#some-form > input').forEach(i => i.classList.add('error'))

If you want to make it a little less verbose:

   const $ = document.querySelectorAll
   const addClass = (className) => (item) => item.classList.add(className)
   $('#some-form > input').forEach(addClass('error'))