1-2-3 and 4-5 Roku

Wikipedia

The Kitashirakawa family is a friendly family of seven. When their father, a detective, is transferred, they move to a housing complex in Tokyo. On their first day at the new school, the second daughter, Saegusa, and the second son, Shiro, discover their beloved dog, Roku, being bullied in the schoolyard! It seems he has followed the family here. They want to live with their beloved Roku again, but they can't keep dogs in the housing complex... So the two of them make a pinky promise to take care of Roku, without telling the family!!