Bang! You're Dead (1954)

Two small boys are playing in a wood. The younger boy has a revolver and, not understanding that the gun differs from his toy pistol, plays 'highwayman' on the road and holds up a cyclist; the gun goes off, killing the cyclist. Both boys are unaware of the tragic consequences of their game. The body and the gun are found by Bob Carter, who had recently quarreled with the victim in the presence of their workmates, and both men had uttered threats. The evidence is strong, and Bob is arrested for murder...

Unfortunately the movie Bang! You're Dead is not yet available on HBO Max.