Is it Islamically permissible for my sister to work as a kindergarten teacher for non-Muslims in a church, and what are the potential caveats, if any?

If her work is free from anything forbidden, such as teaching their false beliefs which are impermissible to teach, and if it does not involve assisting them in anything forbidden, and she does not fear that her presence among them will affect her faith or conduct, then there is no harm in it. However, it is advisable to seek another job, due to the suspicion that might arise from her frequenting the church, and due to the reprehensible things she might witness.