You can file an amended return to claim a child who has not been claimed by anyone else, but only if you are legally entitled to claim the child as a dependent. You can use the tool at the following link on the IRS web site to determine whether you can claim the child as a dependent.
Whom May I Claim as a Dependent?
If you are not the custodial parent, you will probably need a signed Form 8332 from the custodial parent in order to claim the child.
