You have that advantage of having the birth certificates and the youngest child's should list all children of the marriage both living and deceased.
There is no marriage in Tasmania on my resource and only the following events were registered in Victoria.
Baptism
DAVIS William James
Father William Mother Ellen EGAN
At Geelong 1851 Reg#5886
Denomination Roman Catholic St Mary's Geelong Fiche#76
Death
DAVIS William James
Father William
Albert Mother Ellen EGAN
1857 7 years Born Geelong Reg#2829
No birth registration for this death
DAVIS William James
Father William
Albert Mother Ellen EGAN
1860 1 year Born Collingwood Reg#4994
Birth
DAVIS Mary Ellen
Father William
Albert Mother Ellen EGAN
At Melbourne 1864 Reg#3533
Birth
DAVIS Catherine
Father William Mother Ellen EGAN
At Carlton 1866 Reg#16250
DAVIES Frederick Arthur
Father William
Albert Mother Ellen EGAN
At Melbourne 1869 Reg#16956
DAVIS James Edwin
Father William Mother Ellen EGAN
At Melbourne 1872 Reg#10421
Geelong Advertiser 26 Jan 1854
http://www.rootschat.com/links/0yaz/Before the Mayor and Inspector of Police.
ASSAULT.
William Albert Davis was placed in the dock, charged by Mr F. Carman under the following circumstances. ..........
Cando