Unfortunately the St Helier baptism records only record the baptisms of 3 of your grandfather's siblings, Hilda Louise 1893, Walter Ambrose 1890 and Charles Edwin 1891. There are 4 children with the family in 1901, Walter, Charles, Arthur and William, it's possible that Hilda died.
The CI census is merely the Channel Islands census returns, in 1871 John Ambrose Fox is aged 7 and living in Byron Rd, St Helier with his parents William and Anna and siblings Charles, Mary, John and Walter. There are also 2 half siblings, Emily Amelia Darling and Henry W Darling. Going back to 1861 there are other Darling children, Alfred and Anna. William and his wife Anna were both born in England as was Anna's first husband, Felix Darling.
Margaret Mahoney, the mother of Arthur William was born about 1866 in Jersey, her parents were John and Catherine Mahoney, both born in Ireland.