Can I file a motion to Amend my divorce decree? So he can't claim the children on his taxes

It's probably not necessary. The court always retains jurisdiction over child support, which is always modifiable. Who gets the dependency exemptions is a child support issue and goes along with it.

A motion to modify child support requesting an order on who gets the dependency exemption is all you need. Whoever has the kids greater than 50% of the time gets the dependency exemption as a matter of law unless there is a release, which can be voluntary or ordered by the court. If you have a true 50-50 split on custody, the parent with the higher adjusted gross income (AGI) gets the dependency exemptions unless there is a release.