f your allegations of abuse are real, you need to call Childrens Protective Services or an agency similar to that in your State to investigate and document with a Children's Doctor or Psychologist.
My name is Stephen R. Cohen and have practiced 39+ yrs. I can be reached at 213-819-1171. I practicein Los Angeles and Orange County, CA. I give only appointments over the phone., these services do not create an attorney client relationship. My answers may offend as I do not believe in pulling punches or sugar coating the truth, I use common sense as well as the law. Other state's laws may differ and I would appreciate it if you feel like marking my answers helpful or best. There are a lot of really good attorneys on this site, I will do limited appearances which are court documents , less expensive.
If the caregivers (father and grandfather) were arrested for hitting your minor children, the Department of Children and Families (DCF, formerly DSS, the Department of Social Services, which is the Child Protective Services agency in Massachusetts that Atty. Cohen mentioned) should already be involved. Talk to the DCF worker about helping you to get your sons out of that abusive situation.
You can also call the police for the town your sons live in or have them call the police themselves (or DCF) if there are additional incidents of unreasonable hitting or emotional abuse.
Since you had a lawyer when you filed for the custody hearing, talk with him/her about what advice s/he can give you for the upcoming hearing. You should expect to pay for the lawyer's time, but it shouldn't be as much as for representation at the hearing.
If your income is low enough, you could use the Find Legal Aid link below to see if a free program for representation of low income people is available to you for this kind of case. You could also consult with the Lawyer of the Day at the Middlesex Probate and Family Court.
If the lawyer you had for your hearing doesn't handle Limited Assistance Representation (LAR) cases, check out the LAR link I have included below.
How to fight for custody is a question that takes more back and forth and more information than can be handled in a public web forum such as this.
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Do it right the first time which means find a lawyer to help navigate the custody issue. However, except for the 3 months ago incident most of this sounds a little out of date to effectively help you.