I have found Stephen and Amanda/Mandy in the 1900 census...

1900 Census Portland Twp. Ionia Cty,Mi
Steven Denny 63 yrs old Born: Canada Nov 1837
Mandy wife 57 yrs old Sept 1842 Married 28 yrs/ 38 yrs
The marriage yrs were hard to read
7 children 5 still living

Then here is George

1900 Census Ionia Twp, Ionia,Mi
George Denny 1869
Gertrude
William son
Leroy son

Hope this helps so Stephen and Amanda died maybe between 1900 and 1910...