Depending how far you want to take your search, Richard was listed in the household as son in the 1930 Buchanan Co MIssouri CensusWashington Twp. Joseph 35 Head of household Minnie37 Wife Richard 11 son Mildred 11 daughter (wonder if they were twins) Marjorie 10 daughter
According to the SSDI, Joseph Knapp died May 1979. If you contact the public library, maybe they can help you get an obituary that will list survivors. Through them, you may be able to find Richard.