Hi. I searched the 1940 census (currently free and available at the Ancestry.com website) but didn't find your grandparents. You no doubt have more information about them, such as Walter's date of birth, and perhaps you'll be able to find them.
An online tree for a Walter Marteen Pedersen says that man was born 1893 and lived in Dawson County, Nebraska.
This appears to be a Find a Grave memorial for that Walter Marteen Pedersen. The wife he is buried with is not named Florence:
In other words, based on the information you've given, something is not adding up. More information from you is necessary for a volunteer to help you. Or perhaps you are content to wait until some relative may be searchin g the internet and find your post.
You probably know the names of the children of Walter and Florence but simply did not post them; are you seeking contact with the grandchildren? You might be able to find them on Facebook.
I share your reluctance to post information on living people, and so I understand the brevity of your post.