There is a great deal of information in your posting which makes it difficult for an attorney on this site to provide a clear answer. First, if the father of the children has court appointed representation and this is a Juvenile Dependency case, the mother of the children would also be entitled to court appointed representation. My guess is either the mother has not appeared in court or the mother's attorney was relieved since the mother stopped appearing in court. It appears that the court ordered the father to complete a reunification case plan to see if he could reunify with the children. It is unclear whether the mother was ordered to complete a reunification case plan (its possible that the court did not order her to complete a case plan because of a number of reasons including that she has prior CPS history, etc).
A De Facto Parent application if granted would give that person the ability to participate in the proceedings in a limited manner. Overall, you should consult with an attorney about this situation.
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So these are your Mom's grand kids? That explains why she does not have a place in the court room. Have the kids been taken from the father? If so did he try to get the kids placed with your Mom and the Court refused. Anything to do with CPS is complicated, add to that the lack of facts in your question and it is impossible to give an informed response. Contact an attorney in your area immediately.
Note this answer does not constitute legal advice, and should not be relied on. Each situation is fact specific and court specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship