Alexander Whitfield married Kate Newson on 23 3 1904 in Norwood, SA. Alexanders father's name is Henry Alexander Whitfield as you know.
In the SA births there are two registered children from Henry Alexander Whitfield and Mary Anne Clarke.
One is Mary Victoria born Oct 3 1881 at Glenelg (Symbol 'S' says more info on the certificate)
The other child registered in SA but born in NSW is Lancelot Thomas born 7 Jun 1879 born in Poliac NSW. there is a symbol D on this one for(difficulty deciphering registrars handwriting)
I had a look at NSW BDM at their website and found Henry Alexander Whitfield and Mary Anne had other children William H born 1877 at Wentworth NSW --- In the SA marriages there is a William Henry Whitfield married Amelia Grace Herbst in St Paul Church Adelaide (Same church as your Alexander)on 29 April 1903. father is Henry Alexander Whitfield.
Other children for Henry Alexander Whitfield and Mary Anne in NSW Wentworth are Edgar born 1880 died 1880, Barbara E born 1884.Looks as though Mary Anne died in 1885 at Pooncarie Wentworth and then Henry remarried Clara E H Davis 1886 there is at least one other child 1891 in Wentworth from this marriage on the NSW on line database.