HI Andrew,
Sorry that I have missed your email earlier. I am not sure I understand your question correctly.
In order to renew for the GV, your kids must filful the 3 criteria above. for education as long as the school’s declaration letter say they are studying, it doesn’t necessarily need to have to mentioned full time.
They don’t need to be under bachelor or master program. There are also many short programs in the world that they can take. Many consider a 15 - 20 hours / week program is a full time program - for example studying Portuguese in Portugal. There is always a way to make it happen and not lost the renewal.
After the 2nd renewal, they only need to renew again at year 6, at that time, they can choose to renew into a normal permanent residency program which doesn’t have the requirement of being single, dependent and studying.
I hope that clarifies your doubt.