Hmm -could be either. Quite possible for a 15 year old to have a child but there seems no record of the birth/death of the child Elizabeth she puts on the 1904 birth certificate.
I'm assuming you've seen the records of Hubert's admission and death at the Liverpool Asylum for the Infirm and Destitute:
CHARLES, Hubert Gavin, 30, religion RC, weaver,
Married to Ida Kairns (sic), born Parramatta, last abode Parramatta,
Children: Alexander 4, Hubert 2, both with mother
Date of admission: 22/09/05 Died: 9.45 pm 18/11/1905
Relatives: Wife, Albert St Parramatta; Mother and brother George Church St, Parramatta
Remarks: Father, Alex'r - under this is something I can't make out except for the word Parramatta and what looks like "Gaol"
Mother: Elizabeth Gavin
I see some other mentions on Ancestry. Happy to transcribe but you probably have them.
Edit to add OP added this I think what is say's under the Dad's name Alexander most probably say's "Stonemason, overseer of works on Parramatta Gaol".
Judith