I don't know what would happen exactly...
My DF has raised my girls but they are not biologically his.
I would hate nothing more than if they went to my ex. He has no contact (my choice, no orders in place) and is not a safe person for them to be around.
I know my family and DF would fight tooth and nail though and I'm sure they would come best off as I doubt they would put my DD's in an unfamiliar situation that wouldn't be in their best interest.
Still something worrisome to me though.