From what I have read it is better if only the native speaker speaks the other language. So if you speak english and your DH speaks spanish then they will have no problem learning but if you speak non-fluent spanish and/or your DH speaks some english and some spanish it is more confusing for them. You can look at the playgroup australia website as there may be some bilingual playgroups near you. Otherwise you may find a kids language class. In Brisbane there are alphatykes who do teach spanish. Not sure is Melbourne though.