I would be very, very, surprised if you can get to Walnut Grove without hiring an auto.
@Sully_mn unfortunately we don't drive, do you know how much does it cost to hire a car with a driver or hire a taxi for the day?
We are a long term traveller who quit their jobs to travel around the world, meaning that we are on tit budget, so this could be the last resort or option we have!
Walnut Grove is truly a "little town on the prairie" or perhaps one could say tiny town. I was thinking Marshall might be the closest town nearby with bus service, but it's still more than 40 miles away, northwest of Walnut Grove. There might be the possibility of local tours during their popular months, June-August. You might want to contact the town site and the Laura Ingalls Wilder museum for tourism information.
Thanks a lot everyone, seems that a taxi will be the only option.