I understand this is difficult and emotional for you, but you have to understand, the courts do not just remove custody from someone because of mental health. They have to pose a harmful risk or have to have harmed the children. Her having medicine at your house does not mean anything. Her being ill does not mean they will remove the children from her.
You do not get to decide what are satisfactory living conditions. While that certainly may be crowded, it does not mean the courts will view that as a dangerous situation for the children. You not paying child support is not going to help. There is a reasonable arguement by you failing to support the children that contributes to what she can or cannot afford to live in. Also, you getting court ordered visitation does not mean you would not have to pay child support.
It is not kidnapping. She is the parent. You have no custody. You have zero legal rights because you have not been through court to be given rights to the children. At this time, she has sole legal and physical custody. She can go where she wants with them.
The other issue you have is....you have to file in the area that has jurisdiction. Because her and the children have been gone many months, wherever they are is the area/courts that would have jurisdiction. It would not be where you are.
You have to have an attorney handle this for you if you do not know where/what to file.
You should at least consult an attorney in her area and discuss what your rights and options are. A attorney there is best versed.