The character has the option to drink alcoholic beverages, drink too many and an addiction will occur... but this can be avoided but simply not drinking alcohol
Children can optionally be killed by the player, although there are severe consequences and no benefit to doing so. It is impossible to kill children in the UK and German localizations.
No, but there is a scene near the end (not avoidable, but you can avoid having to see the dialogue very easily) where the protagonist hallucinates a "dead friend" - it's not specified who and nothing is shown on-screen, but a fairly graphic description does appear in the text box.
It's a bittersweet ending. You save California from being overran by the mutants, but the Overseer of your Vault banishes you into the Wasteland in fear that the other Vault Dwellers will follow your example and leave the Vault. In the sequel, it is revealed that other Vault Dwellers left with your character in protest and together they founded a village.
Kaz is thinking of Fallout 4, there is no nuclear explosion at the start of the game, although there is a black-and-white image of one in the intro. The canonical ending involves the player detonating a nuclear weapon in a late-game quest.