Mary Ellen's death shows her as Ellen Horton Died July 27, 1937 as you said birth shown as July 28, 1850 Goderich Township
So she died one day before her birthday. She was 87 Son Alex Horton was the informant Parent's name shown as not known
But since she was born in 1850 ON she should be in the 1851 and later census records.
I could not find marriage for Frederick and Mary Ellen but there is this Osbaledeston marriage
William married in 1878, Huron Co. his parents William and Mariah He was age 21 so born about 1857
1861 GODERICH, HURON, CANADA WEST GRUMMET John, farmer, age 48, born England
Emma, age 48, born England
Mathew, age 23, born Canada
Dan, age 17, born Canada
Anne, age 16, born Canada
Jesse, age 9, born Canada
OSBALDESTINE Alex O., age 17, born Canada
Mary E., age 10, born Canada
William, age 3, born Canada
so it looks like they were not with their parents - who perhaps had died? as the census page does not separate families I am not sure if they are actually living with the Grummet family or if Alex is head of family but thought I would put both just in case ** William matches with age of the William I found in marriages
I looked for a marriage for Alex and found one which shows Alexander Osbalderton age 29, married 1900 Huron - his parents Alex Osbaldeston and Mariah Rapson so perhaps a son of Alex from 1861 census
I am having a problem finding the Osbaldeston family in 1851 to confirm parents.
I am wondering if the marriage record for William has his parent's names wrong as William and Mariah - because I found a site - www. huroncountry.ca/library
which has the 1842 town assessments and for Goderich there is only one Osbaldeston listed and his name is Alexander
OK it get more interesting - there is a tree o genealogy.comthat shows Alexander Osbaldeston the 2nd, born Aug. 14, 1844 Huron County ON, son of Alexander Osbaldeston and Emma Rye He is shown married Oct. 5, 1868to Maria Rapson.
They had children Alexander, 1871 (fits with marriage I found) Audrey, 1873, Herbert 1877 and Mary Ellen/Nellie 1880
This tree only shows two children for Alexander and Emma - Alexander (1844) and Emma (1842) they give no sources so they may be missing children They also do not show any birth, marriage or death info for 1st Alexander or wife Emma Rye
So a bit of a mystery still. From census it does look likely that Alexander (1844) Mary E. (1850) and William (1858) are siblings. If that is true, then if we could find deaths for Alexander and William they may show their parents names to confirm if the tree is correct.
there is another tree on ancestry.com that shows Emma Rye as married to Alexander Osbaldeston and says they had 6 children: Maria, Henry, Susanna, Emma, Mary Ellen and William - no birth years shown and they don't include Alexander int he children
It shows Mary Ellen married Frederick Horton so that does match with your info but still no sources to prove these are the right parents
They have Susana married to John Horton - I see there are some Horton marriages which show these two as parents
William married to Mary Ann Jones - which matches the marriage I found except marriage shows Wm's parents as William and Mariah , not Alexander and Emma so???
Emma married to John Rapson - matches what I found I'm ON marriages - but she is shown as Emma Oswald - parents Alexander and Emma
Henry - they show no marriage for him Maria - they show married to a William Dixon - I didn't find their marriage
so more info, but no absolute info to prove the connection
Find a Grave has burial of William Osbaldeston, born 1858, died 1947, buried Fort Saskatchewan cemetery, Fort Saskatchewan, Alberta - but no parents names shown.
Maybe someone else will have some luck in finding the family in 1851 census which should prove parents of Mary Ellen for you.