Yes, I'd recommend to consult with a family law firm in MA to discuss options. Also, as context, this is a brief background on changing child support in NH. It does not substitute for specific legal advice for your particular situation, nor does it apply to MA, but it may be helpful to provide a little more info.
Yes, agree w/ Anna. Please consult an attorney for further guidance.
More generally, as it pertains to visitation and not merely power of attorney, below please find some info that New Hampshire grandparents have found helpful in the past in re: finding a way to ensure they are playing an active role (with visitation) in their grandchildren's lives
See below (hyperlink). I'm a Manchester-based divorce attorney and I've actually written a blog post about this. It may be of help. It does not substitute for legal advice from a lawyer intimately familiar with your case. But it may be a good start. Feel free to email if you have further questions.