is there a reason why your son does not seem to be involved in his children's care, residency etc.
also the children are of an age to express their wishes, could they do so to a judge and/or social worker, so that even if their mother is difficult, their wishes are officially on record. could they at least keep in touch with you by phone, email etc, until they are old enough to visit where they want.
guess the distance makes it difficult as well.
sorry don't know about the procedures.
but you must put your health first, otherwise they might suffer more by losing you at a tender age, or you become disabled, unable to do anything with/ for them.
sorry for your tears. wish you n yours all the best.