You can outfit your groms here, They have kids specific boards from Salomon, Head and Burton, outerwear and accesories.
Murasaki Sports Morioka board store is in the Aeon Mall Minami. (There are 2 Aeons in Morioka). They have a selection of snowboarding hardware and apparel. It is a small shop, so don't expect the same range you find in the stores in the bigger cities.The shop also does waxing and tuning.