If I understand your question, you had a DCS case that was closed with your children in your mother's legal custody.  If so, I don't think you have to go through DCS to try and regain custody, and I'm not even sure it's possible.  You'll probably have to file a motion, with the court, for a change of custody.

However, you can be absolutely certain that the court will want to know what DCS thinks of the idea, once you file that motion, so it might make sense to go to DCS first and ask them what they would need to see, to approve you as the children's custodial parent again.  If you can't satisfy them, your motion is a long shot at best.