The Buchanan County court would have those records. The Northwest Missouri Genealogical Society also has some guardianship records, however, I don't believe they have them for that date range. You might contact them to ask though. firstname.lastname@example.org
They would also have probate and/or will that might help.
You also might try sifting through the newspapers, as there should have been a legal notice for whomever was appointed the executor of the estate . . . the St. Joseph newspapers can be found online at www.news.google.com/newspapers#S
- then scroll down and find the one you want, alphabetically.