File your 8332 for the other child if you haven't done so already.
Send in your tax return claiming your child as written in the order. Include a cover letter explaining the situation and include all documents to prove your claim (divorce decree and anything else you have). Send this in by mail, not online.
The IRS will figure it out for this filing, but it may be necessary to file an enforcement with the family court and request damages for any dependency reduction they receive.