While there are a lot of matters that you can agree on with your soon-to-be spouse in a prenup, agreeing that you will not have a custody battle is NOT one of them. And, during a marriage, you also cannot contract on matters relating to visitation, child support or custody. Rather, the only time that this can be agreed upon in Texas is during a pending divorce.
It may seem like it would be very far, given the hardship and expense of a custody battle, that you could agree with your spouse on this in advance, but Court’s have consistently held that no contract as to these sorts of terms will be held as enforceable.