Is there any hard sleeper for D type train? Is the child 8 years of age half of the fare?

Yes, usually there are hard sleepers on D type trains. And the children with height between 1.2meters and 1.5meters could enjoy half-price tickets. Children below 1.2meters even could take the train for free if they are accompanied by adults and do not take extra berths. Besides, what is your arrival city? Or, you could check the train schedules and departure station name on the web by yourself with your exact departure and arrival city.