I have just found a record online that has Frederick and Ann Sparke, and Ellie (Ettie) aged three months, leaving London on the 14th July 1875 heading for Bluff.
So it looks as if Ann's other child Eliza, then aged three, was left behind in Essex. I guess she, perhaps, wasn't fathered by Frederick Sparke.
I am not sure, at present, who the 1807 William Ekers burial relates to.
Three children are now known for parents William and Elizabeth Akers:
William baptised 1st May 1808 at Kirby
Susan baptised 13th September 1812 at Kirby
Martha baptised 11th October 1815 at Walton
Son William got involved in what became known as the Kirby Riots, in 1830, and was transported to Tazmania for seven years for breaking a threshing machine at Walton.
Daughter Susan gave birth to a son Joseph Akers, baptised in 1831. She went on to marry Martin Fairbrother and they had eleven children at Walton.
Daughter Martha gave birth to a daughter Louisa Barker Akers, baptised in 1831. Martha married Luke Barker. They moved to Suffolk and had a large family.