Yes, virtually every player in a major conflict almost inevitably does, and the Second Sino-Japanese War/WW2 was an exceptionally large and harsh one.

Probably the best example, while it wasn't exactly a deliberate war crime in an attempt to halt the Japanese the KMT opened the dikes to the Yellow River in 1938. This caused a massive flood that killed hundreds of thousands and displaced millions more, without even particularly damaging or slowing the Japanese. Also it was rare to take Japanese soldiers alive but the brutality of their invasion meant if they were captured they could often expect little mercy from Chinese captors. Furthermore in instances where Chinese forces came across Japanese wounded they most likely killed them or left them to die, both out of spite and material concerns (the Chinese often had barely any medical supplies for their own troops). While I'm not sure of the extent children served Chinese forces and in some cases were likely pressed into service at gunpoint.

Japanese atrocities in the Sino-Japanese conflict are more well known, but as this was a huge war involving both guerilla units and more conventional forces there were undoubtedly many instances of war crimes also committed by the Chinese - things like terrorizing populations and torturing/executing enemy collaborators. Not only were they desperate, it's worth noting that the KMT and CCP were both rather brutal regimes even before the Japanese invaded.