How about boat cruise in Geibikei? The view is fantastic and you will get to throw few "lucky balls" in middle of tour so children may enjoy it.
Also you should try "Wanko Soba", it's a style of eating soba, where a staff stand right next to you to pour cups of soba continuously. It is fun to challenge how many cups you can eat. (And the soba is actually pretty good. ) The restaurant is called Azumaya.
The most famous sight in Iwate Prefecture is obviously temples in Hiraizumi but children at age 10 might get bored. There is one interesting shrine in the Hiraizumi temples area which focuses on "god of eyes". The Ema (wooden plate) is pretty unique and interesting there.
Iwate is famous for "Maezawa Beef", a brand of wagyu beef, so eating Yakiniku or steaks would be nice, too.