“But he hit me first” is considered childish because children are supposed to go to adults with problems like that rather attempt to resolve conflicts themselves through violence. In this situation there it’s no analog to adults who can step in and resolve the situation, so your analogy is a bad one. People have a right to defend themselves using measures proportional to what’s used against them, and thanks to Russian’s actions do far, there’s basically no response at Ukraine’s disposal that would be disproportionate.