trixie, I think you are trying to over engineer a result for your dc/gps in a situation that is completely unforeseable and subject to too many variables.
You don't have a right under the law, even with the guardian's consent, to make them take up guardianship and if they do, not to move.
By trying to impose a moral obligation now, with stipulations on emigration, you might actually be disincentivising them from accepting guardianship, when they might the best guardians for your dcs, albeit in Australia, if and when that decision needs to be taken.
The grandparents would not want to be isolated from your dc, but then again, they do not want for you and dp to die either. I am sure they would want what is best for their grandchild at that time in what would be horrific circumstances, even if that meant the grandchild would live miles away in a different time zone.
There will always be ways around that. Alternatively, you could bequeath some money for the guardians to fly over with their family/your dc.