It depends on the circumstances of the case. Was she convicted of the abuse? Does she have anything else on her record/in her behavior that make you uncomfortable with her caring for your son? You might be able to either 1) obtain some kind of restraining order on your child's behalf, or 2) request a custody modification giving you primary physical custody. You should discuss these questions with a Colorado family lawyer, as the rules are different in each jurisdiction.
The foregoing is not intended to be specific legal advise, but rather general information. If you require specific legal advise, you should retain a qualified attorney in your area.
There is nothing you can do on your own to address your concerns. If you are gong to get anything changed it will require either the cooperation of your ex or intervention from the Court.
Assuming that your ex will not address your concerns and make child care arrangements that you are comfortable with, what you can do is to bring the matter back to your divorce court. This will likely need to take the form of a motion to modify parenting time. You will need to convince the judge that circumstances are such that it is not in your child's best interests to be cared for by your husband's girlfriend. You are probably asking for the 50/50 custody arrangement to be changed since the current arrangemetn results in your child spending significant time with the convicted (?) child abuser that concerns you.
I understand that you don't want to 'wait and see' if there is a real threat and would like to ensure that the 12 year old arrest prohibits your ex's partner from being alone with your child, but you need to realize that the judge may or may not be persuaded. A lot will depend on the severety of the prior charge and whether you can point to anything more recent that gives rise to some concern.
If the issue is important to you and you cannot resolve it by talking with your ex, you have to bring the issue before the Court and you may want to retain an experienced family law attorney to assist you.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.