Wife Anna Margaretha Vogel Engelhardt, age 47, second wife
Child #1 Philipp Andreas Engelhardt, age 25
Child #2 Martin Engelhardt, age 19
Wife of Child #2 Elisabeth Nul [Nolde] [Knoll]
Child #3 Philipp Engelhardt, age 9
Child #4 Friedrich Engelhardt, age 6
Child #5 Elisabeth Engelhardt, age 15
He is not specifically named in the 1834 census, but there is another brother named Michael, who was born about the same year as Friedrich. Michael's name is not listed on the 1798 census. Since there was already an older brother named Friedrich Andreas in the family, it's possible the younger Friedrich was actually Friedrich Michael. In the 1857 census, the head of the Engelhardt household was named Philipp Michael, born about 1793, which just muddies the water since he could be either Philipp or Friedrich from the 1798 census.
I'm including the 1834 census entry below for research purposes.
Head of the Household Philipp Andreas Engelhardt, age 61
Brother #1 Johann Martin Engelhardt, age 36 in 1816, deceased 1831
Wife of Brother #1 Anna Elisabeth Engelhardt, age 56
Child #1 Johann Friedrich Engelhardt, age 33
Wife of Child #1 Maria Barbara Engelhardt, age 29
Grandchild #1 Anna Katharina Engelhardt, age 8
Grandchild #2 Friedrich Engelhardt, age 4
'Grandchild #3 Christian Jakob Engelhardt, age 1
Child #2 Johann Michael Engelhardt, age 24
Child #2 Johann Wilhelm Engelhardt, age 15
Child #2 Johann Jakob Engelhardt, age 13 years 6 months
Brother #2 Michael Engelhardt, age 41 [Missing from 1798 census, unless he was really Friedrich or Friedrich Michael Engelhardt, since his birth year, 1793, is within a year of the Friedrich Engelhardt born in 1792 and named in the 1798 Grimm census. 1857 census suggests his name may have been Philipp Michael.]
↑The 1775 and 1798 Census of the German Colony on the Volga, Lesnoy Karamysh, also known as Grimm; Published by the American Historical Society of Germans from Russia, Lincoln, Nebraska, USA; Published date: 1995; family #145 in the 1798 census, Philipp Engelhardt family.
↑1834 Census of Grimm in the District of Saratov, Russia, Dated 2 February 1835, Revised Edition; Translated by Brent Mai, Concordia University, Portland, Oregon; Published by Dynasty Publishing, Beaverton, OR, USA; Copyright 2011; page 70, family #231, Philipp Andreas Engelhardt family.