How to I re-establish custody with my children?

Your question makes no sense. If the court took away custody permanently you can't reestablish it. If the court set conditions before you can see the kids, you have to show the court you met the conditions. If you simply haven't seen the kids and left them with a 3rd party, custody is not impacted and you have a right to the kids unless the 3rd party has filed something.

Engage an attorney to enforce the order.

If you have visitation provided for in your divorce decree and you have documented that your attempts to see your children were denied-than you can file a motion for contempt to enforce the visitation. If you hire an attorney, you would ask that your attorney fees be reimbursed also.

First, re-read the divorce decree with care. Then, hire an experienced family law attorney to assist you with the process. Every state has different laws, and many counties have somewhat different practices. You do best with a local lawyer who knows the local people and practices. Typically, you would move to amend the order or judgment. The general practice is to move you back into the children's lives rather slowly, perhaps with few hours of placement now, and increasing perhaps by the month if everything works out.