Osaka/Kyoto to Hiroshima

Is it possible to do a day trip from Osaka or Kyoto to Hiroshima or would we be wasting our time with travel etc?
Thanks

It is easily possible, but if you also want to see Miyajima at a leisurely pace, I recommend an overnight stay:
http://www.japan-guide.com/e/e3404.html?aFROM=2157_2160

I would recommend too an overnight stay - that's what I planned.

Here you can plan for the train travel from Kyoto. For Miyajima enter Miyajima guchi http://grace.hyperdia.com/cgi-english/hyperWeb.cgi

For one way ist needed approx. 2,5 hours

As long as you're prepared to get up early, there's no reason why you can't do it as a daytrip. Go straight to Miyajima for half the day in the morning and then back to Hiroshima to check out the museum (allow 1.5 hours at least as there's lots to see at the museum), by late afternoon see the Dome and wander around the park, then get a train back.