A lot of people are essential workers and many of them deal with the public and thus will have higher exposure risk. While I understand your concern, I don't see a court saying that your ex can't take your kid to visit his cop girlfriend simply because her job has contact with the public. That's effectively punishing those who have to do these essential jobs and the people that are close to them, like your ex. Especially considering the next point.
This doesn't make a lot of sense to me. If your ex goes to see her then he has the same exposure risk that concerns you for the kid. And if he contracts the disease then that in turn would put your kid at risk for it. While the risk might be a bit lower if the kid stays home while the ex goes because there is now an extra jump the virus must make, once Dad has it the daily interaction with your kid at home would create a lot of opportunity for your kid to get it. So to really address your concern you'd need to keep your ex from seeing his girlfriend, too, and I think it extremely unlikely that the courts are going to tell him he cannot see her when the only fault of the girlfriend is that she is a cop.
It would be different if the cop was confirmed to have the disease and your ex intended to take the kid over to see her. But your problem is just with the risk that the cop might get it, and every one has at least some risk that they may get it unless they succeed in completely cutting themselves off from others during the entire duration of the pandemic.
But the best person to ask what kind of shot you'd have for a restriction on your kid and/or your ex going to visit with the cop given the exact orders in your case is your own attorney.
Bear in mind too with the restrictions in place for the virus the Washington courts have limited their operations for the time being, so unless you can convince the court that this is truly an emergency you may find that it would take quite awhile to get to trial on this if you wanted to litigate this to get an order limiting your ex.

