Dealing with DHS in this state can be a nightmare I am sorry to say. I highly recommend that the grandmother retain an attorney right away before DHS files a Pendency Neglect case against the parents. When there is a suitable relative, particularly a grandparent to take custody of the children, that is supposed to the be the first preference over foster care but getting them to do that is often a battle. The sooner the grandmother retains an attorney the better for all concerned but most importantly the children.
As Ms. Hudson said, if the grandmother can get guardianship prior to DHS filing their paperwork, DHS will often just drop out of the picture, even the grandmother doesn't beat DHS with the filing, they will often allow the case to stop if there has not yet been a probable cause hearing.
However, if the grandmother is adversarial, and DHS has reason to believe that the grandmother will not protect the children and just turn them back over to the parent, DHS may not give up so easy.
Long story short, get involved fast, but be "nice" about it, and follow the orders of the court, if parents have to get DHS services prior to getting custody back, do not go behind DHS's back, it will not end well.
Every legal matter is fact specific, and there are often nuances in every case. This is intended for comment only, and does not create an attorney client relationship.