How can an adopted child get an EB-5 green card?

This is a problem insomuch as you are already a permanent resident of the United States on a conditional basis. If you had adopted your child one day prior to obtaining your lawful permanent residency on a conditional basis, you could include your child. Now, as a permanent resident, you may petition for your adopted child, but there is a backlog under the quota. If, after five years as a lawful permanent resident, you become a U.S. citizen, there would be no backlog. Thus, you should see an immigration lawyer who deals with family immigration cases, hopefully Board Certified in the field of immigration and nationality law by his or her State Bar.

Since your adoption was completed after you became a permanent resident, at this stage you will need to file/petition for your adopted child as a lawful permanent resident petitioning for a family-based immigrant relative petition, provided that your child is a minor, and all the adoption paperwork is in order.

If your child is under 16, and adopted pursuant to Hague Convention Rules, you can include the child in your I-829 filing.