A custody plan can be modified by something such as a motion to modify the existing court order. The standard for modification is usually a substantial change in circumstances, which might include violations of the custody plan, making derogatory comments about the other parent in the child's presence, and making false allegations against the other parent pertaining to abuse or neglect.
That last one, making false allegations of abuse, especially sexual abuse
, is especially heinous and often grounds to totally suspend a parent's access to a child. This is because false allegations of sexual abuse are generally considered a horrible form of child abuse
in and of itself. Children are often unable to discern the difference between being a victim of abuse and simply being told or treated as if they are a victim, and consequently the courts detest a parent putting a child through that, as well as any other form of parental alienation
I would suggest that you contact a local attorney as soon as possible, because given the child's age time is of the essence. If you are unable to afford a lawyer, you can always apply with your local legal aid office. They provide free legal representation.