On the other hand, since Australia operates under treaty with the US to return such (potentially) abducted children to the US, you would probably need, at the very least, an iron-clad, rock-solid, court order specifying when the child can go to Australia (full detailed itinerary), with whom she may go, for how long, where she will stay, details for contact info allowing contact with parents during trip, but probably most importantly, penalties and sanctions in case the grandparents flout the court order requiring her timely return.
The court order (allowing travel to Australia) should state clearly that the grandparents agree to pay all attorneys fees and expenses related to any litigation, travel costs, in repatriating the child back to America (not to mention costs of travel to and within Australia), in the event the grandparents breach the contract and/or ignore the court order.
Then, after all that, once you have a court order signed by a family court judge (not commissioner), if you feel comfortable about it, then let the child go to Australia. Undoubtedly, it will be the experience of a lifetime, and if possible, the child will greatly benefit from the experience. However, if things turn sour, it could be the ugliest experience of your life, if you don't have the paperwork in order (and I don't mean the passport).
This is not legal advice, just human spiritual advice, from one human to another.
Good Luck.