Welcome to Just Answer. Please let me take a moment to review your question.
I can certainly understand your feelings of being overwhelmed. This is a complicated situation and one that is going to take some time and some professional help to sort out. It sounds like you are asking two questions: 1.) How does your son manage supervised visitation for his children with his soon-to-be ex wife and 2.) How does he get legal advice and manage paying for all their expenses, not to mention scheduling issues. Is that correct? Since you are offline at present, I will attempt to answer both questions to the best of my ability.
First of all, since their are minor children involved and potential abuse by the mother, the children's safety (and your son's) are of utmost concern. We have laws in the US to protect children in these situations, so I strongly suggest your son gets some legal help asap. There should be a "legal self-help" center in your city; most cities have such departments as part of the court services. The legal advice is often free or very low cost, based on income. Child Protective Services is also mandated in every county and they should be notified of the mother's abuse asap. It is only by reporting this abuse to CPS, that they can help your son. If there is no record of this abuse, it is going to be very difficult to protect the kids and get help from the court regarding custody. It is also the law that abuse must be reported by any professional that learns of it. So, when your son gets legal advice and informs them of abuse by the mother, they are required to report this to CPS. Your son can also get a restraining order from the court so that the mother cannot see them or him, except in a supervised setting.
The family court system in your county will also have free mediation or counseling regarding child custody in the divorce, but they must know all the facts to protect the innocent. In other words, these laws are set up to protect minor children and your son shouldn't be afraid to use them to protect his kids. They cannot protect themselves and depend on the adults to do so. You can even make an anonymous report to CPS if you are concerned about their safety, but it sounds like your son is in the process of taking care of this. I hope this answers your question. I appreciate a positive rating if you are happy with my service. Thank you and take good care.