Is it legal for CPS to speak to my children at their school during recess without me (their parent) present?

This message board is meant for general responses, as opposed to specific legal advice, and it would be difficult to assess this situation without a full history. Given that you indicated there already is an open CPS case you should consult with an attorney who deals in that areas. While the goal of CPS is supposed to be family reunification, you want to be sure that there isn't something going on behind the scenes that could adversely affect you.