(1) A legal parent of the child, as defined in section 1-232 of this chapter;
(2) An adult allocated custodial responsibility or decision-making responsibility under a parenting plan regarding the child that is then in effect; or
(3) Persons who were parties to a prior order establishing custody and visitation, or who, under a parenting plan, were allocated custodial responsibility or decision-making responsibility.
(b) In exceptional cases the court may, in its discretion, grant permission to intervene to other persons or public agencies whose participation in the proceedings under this article it determines is likely to serve the child's best interests. The court may place limitations on participation by the intervening party as the court determines to be appropriate. Such persons or public agencies do not have standing to initiate an action under this article.