Killing children or torturing innocent people are absolutely war crimes. When talking about the Ukraine conflict specifically, those two are actually the most common ones that I think of (in addition to destroying civilian infrastructure like hospitals or power stations).

Ukraine isn’t doing either of those things, on any scale that’s relevant, and I don’t think they should start, for reasons I’ve already touched on. They won’t have all that much of a productive effective on fiercely “fighting the war.” That’s actually exactly a big part of what makes them war crimes (where something like blowing up an oil refinery is not.)