You do have the right to claim the kids if they live with you ... "legal custody" means nothing to the IRS. So you can amend your returns if you like but be aware that you will probably need to prove your claim to a future IRS inquiry since both parties trying to claim the same dependents will both need to address.
For details see:
http://taxes.about.com/od/dependents/qt/Dependents-Audits.htm
