In a situation where the children are still alive, but it's impossible to rescue them, their mother is forced to "get out of there while she can." It's a very heart wrenching scene.
A child is punched in the face early in the film by an elderly man. Later, a disfigured man is harsh towards the kids nursing him at first but gradually warms up to them
During the bomb's blast, there is a very gore depiction of multiple individual people getting progressively disfigured whilst melting away from thermal radiation, until being reduced to ashes. Even if it's "only" an animated scene, it's really disheartening and I would recommend skipping it if anything of the kind would pull you off : happens right after the sequence where you see the pilot holding a watch and Hiroshima from above.
It’s not given specific focus in the scene, but as a family burns alive under the rubble of their home the young boy’s toy boat sits in front of him and is implied to have been destroyed in the flames