1. If the cheques were to be made payable to you & your ex then the two of you are the beneficiaries under the policy - not your daughter. If that is the case, then your ex's stated reason for refusing to sign does not stand up - he could simply keep his share & not pass it on to your daughter.
2. On the other hand, if your daughter is the beneficiary under the policy the insurer should pay her - not you & your ex.
3. If the above is not an accurate statement of the position & your ex maintains his opposition you should go through the insurer's complaints procedure to try to get them to pay out your share regardless of what your ex does. Ultimately, you could go to the Insurance Ombudsman.