The process on Perth airport (roughly, maybe weong order od things):
Everyone from 1 flight gets in line, gets a new mask, sanitise hands. They measure temperature and ask you some health questions.
Then passport check as usual.
Get your baggage.
Interview with police and/or show your G2G pass (I think this is WA specific).
Get assigned a seat until everyone in your flight is on a seat. Then in groups you walk (with your baggage on a trolley) to the bus. Chuck your baggage in there and have a seat. When everyone is in the bus, the next group will walk to the next bus. i hated this part. Because I was in the first bus, had to wait for 45 minutes somehow before the second bus was filled. It was 36 degrees outside and I was sitting in the sun. No aircon could cool me down enough.
When all buses are ready, it's escorted to the hotel by police. At the hotel you get off the bbu, walk into the hotel. Check in and you walk to your room. Which is your home for the next 2 weeks.
You get C19 tested within 48 hours and on day 12. Nurse calls you a few times a week. Food is put in a bag in front of your room and you can leave the bag with waste when you are done. You cam have groceries delivered (coles.com.au or whoolworths.com.au).
Exactly 2 weeks after landing (if you land at 2.32pm you can leave at 2.32pm) you can go.
The child should be able to travel on a 600 or similar short-stay visa with an entry exemption as the direct family member of a PR or citizen. Once onshore they can then apply for PR. But of course this will mean paying for the 101 visa twice.
Update: Given that Home Affairs has announced that 101 visa applicants may be able to obtain the visa onshore, they may not have to pay for it twice:
@Paul1Perth I also thought that when I read the BBC headlines yesterday, but they are just closing the 'travel corridor' so that people coming to the UK will have to quarantine at home for 10 days (5 days if a negative test after Day 5). After a couple of days or so a lot of people are just going to say 'I feel fine' and off they go out. We had the same problem here in the beginning, but at least our government clamped down on it.