It's been a while now since I looked closely at the WA law, but generally, grandparents do not have any rights to their grandchildren. I'm sure someone with more knowledge will come along and explain it a bit better. But given the circumstances, surely those in power must be reconsidering the wisdom of the law.
In this case, it won't matter, if Josh is arrested.
It will then be a matter of three grandparents versus one grandparent:
On Josh's side:
His father, Steven Powell
Representing Susan's side:
Susan's mother
Susan's father
Josh's mother (divorced from Steve)