If your decree is silent, the exemption belongs to the custodial parent. No need to contact Dad about it. If Dad claims her too and the Service challenges you, you simply demonstrate that you're the CP. You win; Dad loses.
most of the time, this is true, Lee.
However - the IRS has it's own rules about who gets to claim the children when both the CP and the NCP claim the child if it is not spelled out in the divorce decree.
Here is the text from publication 504 regarding if 2 people claim the child.
Table 4.When More Than One Person Files a Return Claiming the Same Qualifying Child (Tie-Breaker Rule)
*IF more than one person files a return claiming the same qualifying child and ...
1 - only one of the persons is the child's parent
2 - two of the persons are the child's parents and they do not file a joint return together,
3 - two of the persons are the child's parents, they do not file a joint return together, and the child lived with each parent the same amount of time during the year,
4 - none of the persons are the child's parent,
*THEN the child will be treated as the qualifying child of
the ...
1 - parent.
2 - parent with whom the child lived for the longer period of time during the year.
3 - parent with the higher adjusted gross income (AGI).
4 - person with the highest AGI.
here's a link to the publication.
http://www.irs.gov/publications/p504/ar02.html#en_US_publink100044170