If a case is opened, both parents and the children will probably be evaluated including home visits. The courts are very reluctant to terminate visitation between a parent and child. Therefore, the best solution is to try to assure that the child will be safe during visitation. Your word is not enough. Obtain the driving record and criminal record of the offending parent.
Nothing speaks louder than a history of drunk driving offenses.
The more recent the evidence the better. Consider hiring a private investigator. This is expensive, but photos or videotapes, while usually not admissible in court, can lead to an agreement with the offending parent to be taken to the court. Have an unbiased witness with you when the child is being picked up or returned home. This witness should document observations and be willing to testify in court.