Your best source for the children of Alexander Dieffenderfer is the Lehigh County (PA) Historical Society. Their web site says they have the records of the Great Swamp Reformed Church, although they show the records they hold only date back to 1963. They may be able to help with earlier records.
My record shows that Alexander was confirmed in the Great Swamp Church, and his brother Gottfried (my ancestor) was married there. However, Alexander married Gertrude Leydich in New Goshenhoppen, Montgomery Coutny, PA. I'll try that in my search engine.