Travelpass for Tokyo-Kyoto-Hiroshima-Tokyo?

Hi! We are a family of five that want to travel with Shinkansen from Tokyo, via Koyto, to Hiroshima and back all the way to Tokyo again. Should we buy a travelpass for this round trip?

You should get the JR Rail Pass, described here:

http://www.japan-guide.com/e/e2361.html

It is cheaper than buying all of those tickets individually.

travel with Shinkansen from Tokyo, via Kyoto, to Hiroshima and back all the way to Tokyo again.

If you travel so within 7 calendar days, a 7-Day Ordinary-Class Japan Rail Pass will be cost-saving.
If you allot more day(s), regular fees may well result in saving a cost compared to a 14-day Ordinary-Class Japan Rail Pass.
: If you travel from/to Tokyo to/from Hiroshima within 6 calendar days, each of you can pay a base fare for the whole route, not separately per a part of it.
: If you travel to Hiroshima and travel the way back to Tokyo within 12 calendar days, a roundtrip discount (by 10 percent) will apply to the base fare for each way.
: Shinkansen charges are calculated by sections (Tokyo -- Kyoto, Kyoto -- Hiroshima and Hiroshima -- Tokyo).
: You can travel further to Miyajima Island at almost the same cost.

By the way, which airport(s) are you going to use?

Thank you so much for the useful information. Does this meen we have to buy the railpass for whole of Japan, or can we buy one or two zones? We will land at Narita airport in Tokyo.

For the whole of Japan. The different regional passes will not work well for you as you move across multiple regions/companies (JR East, JR Central, JR West)

JR Pass info
http://www.japan-guide.com/e/e2361.html

Enjoy your trip!

How is your full itinerary? Will you leave Tokyo and get back within 7 days? If not, check whether a Japan Rail Pass will save your cost. The price of a 14-day Ordinary-Class Japan Rail Pass may well be higher than a total of JR regular fees.

can we buy one or two zones?

No.
JR-Central, which is operating Shinkansen between Tokyo and Shin-Osaka, is issuing no rail pass like a JR East Pass.
Please note that with a Japan Rail Pass, you cannot choose a Nozomi train on Shinkansen.

Enjoy your trips!