Take the first 2 Trains (6:02 or 6:35) to reach Grindelwald by 6:30-7am, you can spend 45-60 mins here and board the 7:47am to KS. I recommend spending more time in KS as its incredibly scenic. Then take the 8:30 or 9am Train to JungFraujoch!
(Tip : Each station has Luggage Lockers so you can explore uninhibited.)