Grandparents in Tennessee MAY have standing to petition for visitation...the issue is going to be that until the children are in some situation more grounded than temporary protective custody of the state, courts are loathe to take ANY action, at all, until the fires die down a little.
In the meantime, much will depend on what is happening with mom; how well/quickly she works her case plan, etc. The FIRST choice of CPS is going to be to get the kids back with mom, until and unless it is definatively determined that this can't/won't happen. The NEXT choice is placement with other family - like the father, if he is (a) known, (b) alive, (c) established legally as parent, and (d) not otherwise DISqualified from custody, THEN other family, including grandparents come onto the guardianship radar. If you want to be considered on that short list, the best thing you can do is treat the situation as if you'd be taking in ANY child (ie like a foster parent).