With the US approving Pfizer for 5-11 year-olds if Australia follows suit one would think it's only a matter of time before kids aged 5-11 have to be vaccinated in order to travel overseas, and to return home without quarantine. If it's not required in January one would certainly think by February after the school holidays such a requirement is likely to be brought in.
Kids aged 5-11 may not be able to have both doses before Christmas, but they should be able to have at least one if they need to travel. So another possibility would be a staged introduction of a vaccine requirement for kids aged 5-11 requiring at least one dose and then later to have had both doses once enough time has elapsed.
They could also stage it by bringing in the vaccine requirement first for outbound travel then say a few weeks or a month later for those returning to recognise that some may not have had time to get vaccinated before departure.