Right after Jim throws Hilda into the fallout shelter, a scene of their house suddenly becomes purple and blue and then goes black. this could catch people off-guard as it happens really quickly
When the bomb finally hits we see a building being destroyed, sheep burning (not graphic at all), a church being destroyed, a train going off the rails, and a house getting demolished all set to dark music in the background. We hear the repeated line by Hilda of how her cake will be burned.
The movie takes a slow approach to becoming incredibly heart-wrenching. It may be difficult to watch for people of any age, with its theme about the aftermath of a nuclear attack.