If there is a denial on a I-526 petition, you can always appeal. However,in our opinion the quickest process is to re-file the I-526 petition. Please note the reasons for the denial and therefore supply the previously missing documentation in the new filing of the I-526 petition.
The delays may be less as more I-526 petitions may have been approved and deference is given to the project documentation in previously filed I-526 petitions. Of course we are assuming the re-filing is with the same EB-5 Regional Center Project.
There may be opportunities to appeal the denial, but re-applying is always an option. When re-applying you must consider children who have since turned 21 years of age in the time between filing and the denial (as they will no longer be considered derivative beneficiaries). Since this will be an entirely new filing (even though most of the package will be the same) you will have to wait for the current processing time. That is, you will not receive any credit for the original submission.