If you signed Form 8332, www.irs.gov/pub/irs-pdf/f8332.pdf, giving your ex-husband the right to claim the children your mother cannot claim the children in 2008.
A child who receives more than half o his support from his parents during the year and is in the custody of one or both of them for more than half of the year is normally the qualifying child of the custodial parent. However, the noncustodial parent can treat the child as his qualifying child if the parents are divorced, legally separated or lived apart at all times during the last six months and the custodial parent releases the exemption to the noncustodial parent.
There might be tax rebates for 2009 but if you signed Form 8332 for tax year 2008 you must wait one year to revoke a previous release exemption.
Revoking Form 8332.
The custodial parent can revoke a previous release by providing written notice of the revocation to the noncustodial parent. This revocation is effective for the year designated on the revocation, but not earlier than the year AFTER the year the revocation is provided to the other parent. In other words a revocation provided during 2008 can be effective on earlier than 2009. The custodial parent must make reasonable efforts to provide actual notice to the other parent and must keep the evidence of the delivery of the notice to the other parent. It may be a good idea to have the custodial parent mail the revocation using CERTIFIED MAIL as proof of the fact that the revocation was sent and the date it was sent.
The revocation can be made using Form 8332 (as revised) or a written declaration that conforms to the substance of the Form 8332.