I think the only way I can see to get obstetric and paeds experience in Australia,is to actually become a midwife as well as a nurse. That would require either a 1year graduate diploma in midwifery or the ACU 2 year graduate Batchelor of Midwifery for already RN's. Overall probably take 3-4 years to be qualified for the US....if you manage to get straight into midwifery programs after finishing nurse program.
Deakin University had programs in mid and paeds that was accepted by most states of the US...(so I have read on forums) but it had been rejected by the Cal Bon in recent years. Deaking were going to revamp things, they might be worth contacting or visiting the website for the mid and paeds modules.
It also seems (by reading the posts on Allnurse) that it is near impossible to get obs and paeds education to make up for it in the US without doing whole RN degree.
Different idea of what nursing is and what is midwifery. Nursing here in Australia an RN is considered to be general educated, without obstetrics and education in paeds. (RNs work in kids but often have not had special education for kids nursing.)
In the US RN's do obstetrics and have education and clinical in paediatrics.
Women having babies is not seen as needing a nurse but a midwife in Australia and are thought of not being unwell. Just needing someone to support, give education and help deliver their baby. There are even straight degrees for midwives, though these are being phased out as really - you need to be a nurse first before a midwife due to co-morbities women usually have.
If you are happy knowing that it will take at least 3-4 years and lots of extra money to be a midwife well come to Australia, but if you want the assurance that you will be eligible for registration in the US then, educate there as a nurse.
ps, the 2 year Associate degree pathway to RN's in the US is not acceptable anymore in Australia, if that pathway is thought of as another option.