My suggestion is that you first try to SCARE him out of doing this. How? By pointing out the SEVERE penalties that accompany any DUI-DWI conviction when children are passengers in the vehicle. You did not mention which state you are from, but (for example) in Illinois, the minimum jail sentence is 6 months to serve, if convicted of DUI and having a child in the vehicle. Beyond jail, most states (upon conviction for DUI with one or more child passengers) will either enhance license loss (or suspension) or totally take away driving privileges. In Georgia for example, a first offense DUI conviction with a child in the vehicle creates TWO DUI convictions, and your entire driving privilege are lost for 12 months --- no work permit. That case is Dozier v. Jackson, 638 S.E.2d 337 2006). Few people can maintain a good job without being able to drive.
Also, if he injures of kills one of your children (or any other person) he will be facing either vehicular homicide charges http://www.georgiacriminaldefense.com/georgiavehicularhomicide.html, murder or vehicular manslaughter --- depending on how your state frames the felony offense OR "serious bodily injury by vehicle" (also a felony). The penalties range from several years in state prison to LIFE imprisonment in certain states.
Tell him that you will be checking him for alcohol on his breath, and if he has ANY odor of alcohol, he need not come to get the children, because they will not be going with him in the car. Put this in writing, and send a copy to your divorce attorney. If necessary, go to court for a modification of custody whereby visitation is either haled or the husband is required to install an ignition interlock device on the vehicle that will not let teh car start when alcohol is on the driver's breath. http://www.smartstartinc.com/aboutus.htm
If your "scared straight" tactic does not work, and you are certain that he is driving the children around after drinking, I would call this report in to police and have him pulled over. I'd rather see my spouse in jail that live a life of regret after an accident involving crippling injury or death to either of my children. Calbi v. Calbi, 396 N.J.Super. 532, 935 A.2d 796 (N.J.Super.A.D., 2007).
Certain serious or repeat DUI-related offenses can even lead to deportation. Marmolejo-Campos v. Gonzales, 503 F.3d 922 (9th Cir. [AZ], 2007).
An attorney with experience in family court matters should be able to help you. A family court judge may have the authority to order your husband to simply not drive with your children in his car. Many states also have special laws on the books which provide a way for a family member of someone with a serious drug or alcohol problem to go into court and request that the person undergo court ordered treatment. I also think you should consider contacting your local District or State Attorney's Office and speaking to them about your concerns. It sounds like your ex-husband is committing child abuse each time this occurs.