When a person whether it be an adult or a child is coerced to conceal a crime it is a tragedy to today world but it is the reality that we live in. There isn't law per se that protect an adult or child from the influence of others except the normal ones we have (like tampering with a witness) and the conscience of the people involved. Good Luck and pray to Heavenly Father for help here .
Although there may not be an applicable crime to charge the custodial parent with, you may have a contempt case against this parent. Depending on what type of and the frequency of the domestic violence, you may also have grounds for modification of your current parenting plan. I suggest you confer with an experienced family law attorney in private about this and discuss your options.
It sounds as if your disputes with the ex remain ongoing. I am sorry to hear that. This will only hurt the child in the end. How do you know that the child witnessed a crime? How old is the child? Children under 10 are of questionable competence, i.e., ability to know right from wrong, fact from fiction. So, ask yourself, how important is it to get the child involved in these disputes. Only you are in a position to judge the situation and protect the child.
At your service,