In the 1840 census, I don't think the Male age 15 to 20 could be a child of William G., given that William was 28 in 1840.(Families seemed to often have a neighbor or male relative who lived with the family, possibly being a hired hand.)
The Male under 5 years would be John.
The Male age 5 to 10 years would be Samuel.
The Male age 20 to 30 years would be William G.
The Female under 5 years would be Catherine. Depending on what month the census was taken, it's possible that she could have been born in 1840 and listed in that census.That would mean that her mother got pregnant again immediately after John's birth.
I have a lot of Stahl blood in me.I am descended twice from Wilhelm Stall and once from Johan George Stall, who were brothers.Johan George was, of course, William G.'s father.It appears to be his generation when they changed to the Stahl spelling.