The "how many hours" question is difficult to answer because your primary interest is/should be the Peace Park and Museum. Can't really answer how much time you should spend in any park or any museum. You could linger in or speed-walk through the park and/or spend seconds or many minutes at several/all of the exhibits. However, there is a limit to how much time you can spend in the city.

Looks like the earliest arrival time at Hiroshima via fast shinkansen would be 07:25 and latest departure from Hiroshima would be 22:12. The next-to-last return would be at 21:59 and I would plan on that with the later train being a backup.

That would be about 14.5 hours. Subtract 15 minutes spent at Hiroshima Station both after arrival and before departure. Thus, you can have a maximum of ~14 hours in Hiroshima. Figure 2 hours each way for traveling to/from the shinkansen platforms at Hiroshima Station and you have a maximum of 18 hours for your day trip.

If you want an 18-hour day trip, then more power to you. I recommend that you get the earliest train that you are all comfortable with, but either don't book the return trip or get seats on the next-to-last train. This would be to avoid the schedule pressure. You could make/change reservations online (I assume) after you have a good idea on the day of when you want to return.

After the PP&M you can decide if you have time/energy to take the ferry to Miyajima that TBS mentioned or maybe you just want to go to the castle/garden/station.

Miyajima would be the best choice to visit besides, or instead of, the PP&M. That would require a bit more planning than other sights, including checking the tide tables for Itsukushima. You should check the japan-gude webpage to plan your visit.