You don’t flip the coin until the ability resolves. If you want to respond to the ability, perhaps by regenerating the damaged creature, you’ll have to do so before you know the outcome of the flip.

If the combat damage Creepy Doll deals to a creature is lethal, you’ll still flip a coin. If the creature is still on the battlefield (perhaps because it regenerated), it could be destroyed a second time, depending on the coin flip.