The Thing About Luck by Cynthia Kadohata
Summer is a twelve year old girl with a younger brother who is obsessed with legos. Her parents suddenly left for Japan to care for some aging relatives and so she and her brother will be traveling the Midwest with their aging grandparents as they harvest grain for a custom harvesting company.
Summer's grandparents are aging and speak broken English, but have a strong work ethic that they are trying to teach to their grandchildren.
This is a book intended for middle school aged kids, but the story is a little slow and some might give up before they reach the end.