The Onomichi Sumiyoshi Fireworks Festival is a good one, and is due to be held in the end of July (Saturday, July 28th this year), from 7:30pm (see www.bbbn.jp (Japanese) for details) The Fukuyama Fireworks are in mid August, so why not plan a mini break out here, checking out the fireworks, and a few of the places below while you're at it?

Saburo no Taki (Saburo Waterfalls) 三郎の滝 Seen one waterfall, seen 'em all? Not this one! Just off the road between Mitsugi and Fuchu ( National route 486); a stroll into the riverside walk area of Saburo no Taki alone is like walking into air-conditioning at the height of summer, and of course it's very pretty, and they have all the usual restaurant facilities and even a small area where you and your kids can fish in the stream. However the real reason to visit here, and the reason scenes from Saburo no Taki have become a Hiroshima Local TV summer fixture, is this:

Due to formation of the river bed, the waterfall at one point forms the perfect natural waterslide. Every year, as soon as it's hot enough, children and a few daring adults, make it up the mountain to stand in line and body-shoot the rapids. There is a blissfully cooling pool at the bottom to catch you. Take your swimsuit, a towel, and something to sit on while you watch; and I'd recommend going around 12 o'clock, when all good Japanese people are having their lunch (!), to avoid waiting in line for too long.
Last year, we took my daughter, who was 2.5 at the time, and she loved sliding down on Daddy's knee. My daughter is quite adventurous when it comes to water for her age, so I'd say intrepid 3 year olds could probably do it alone, 4 is probably safer, but this is not for the faint hearted.
I never saw anyone getting hurt, but use your judgement! It is great fun for the elementary school crowd!
Grilled river fish, nagashi somen and kakigori await you when you're done, just down the hill a bit!
It's not very clear, but it is red number 4 on the map, which also shows you a few other sights in the area: Fuchu Hiroshima Website (Japanese)

Fuchu Children's Land: POM Just a few kilometers along the road from the waterfalls, is a large children's play centre run by Fuchu City. Fuchu is famous for its wooden furniture, so inside the large indoor playroom, there are various handmade wooden toys, climbing frames and child friendly furniture made by local craftsmen for your kids to enjoy, as well as an array of books. On the top floor of the building is planetarium, and there are various cultural events organised on the second floor. Outside there is a play area with interesting climbing frames, and the river has been landscaped to allow safe waterplay (unsupervised so you need to watch your own kids).

If they're fast enough, kids can try and catch the small fish that swim past in the river too. I recommend sandals for wading, just in case.
See here for details: Fuchu POM (Japanese)
BEWARE: Opening hours are weird: It's closed on Mondays, the 2nd and 4th Thursdays of the month, and the 3rd Sunday! Otherwise open from 9- 5 daily, and you can use the river anytime.