If I am reading into your message correctly, the grandfather has behaved in a way that has made the parents decide to keep a distance for their children's protection.
People in this situation can ask about having a visit somewhere that the grandfather is not going to be present, or perhaps they may need to clarify that they do not condone his behaviour and would be protective of the children. However, if this is a serious matter and they are withdrawing contact for good reason, they might accept gifts and letters via a social worker who can assess whether they would be suitable people to have contact at some point in the future. If social services not involved, solicitors can sometimes help with passing on letters etc.
Last resort is court, and if the grandfather is on the scene at all, as there is a problem about his behaviour, it's very unlikely that a judge would enable contact. Not while grandparents put him before children.
Sorry - this is based on speculation and I might be wide of the mark - it's dificult to gauge what's behind your post without knowing much more.