I need your advice brother. Should the church continue to support a preacher, whose children are already adults but are not Christians?
There are no requirements in the Bible concerning a preacher's children. It may be a concern that a preacher is not able to persuade his own family to be Christians, but it does not necessarily mean he is not worthy of support. For instance, a man may become a Christian later in life and over time decide to preach. His decision does not mean his children will necessarily follow. See:
Don't add qualifications that God did not.