Plan their trips around the subclass 600 visa. Each stay can last up to 12 months from arrival, and condition 8558 allows no more than 12 months in Australia in any 18 months. Change the dates below and everything rechecks instantly.
How it counts: arrival and departure days both count as days in Australia. "12 months in 18 months" is checked as no more than 365 days in any rolling 548-day window, minus your safety buffer. Each stay must also end within 12 months of that arrival.
"Latest departure" finds the last day a stay can run without breaking either rule, given the other stays as entered. "Earliest return" finds the first day they can come back and still stay until the departure date shown.
This assumes no time in Australia in the 18 months before each parent's first arrival, and that the visa is still valid for each arrival. It's a planning aid, not migration advice, so confirm the final dates with Home Affairs or a registered migration agent.