Yes, it should be possible to get a life insurance policy on your ex-husband, but it will require his consent.
Since your ex-husband pays child support for your daughters, you have an insurable interest in him, which is what allows you to take out a life insurance policy on him with you as the beneficiary.
You can’t keep the policy a secret from your daughter’s father since he will need to consent to medical records being released, a medical exam for the life insurance provider and sign papers.
It’s difficult to take a secret life insurance policy out on someone. If your ex-husband dislikes the idea of you being the beneficiary of a life insurance policy on him, you could look into setting up a trust as the beneficiary rather than yourself. That way, the money goes into a trust for their benefit, although you could still be the executor of the trust.
Here’s more on life insurance trusts for child beneficiaries.