Dealing with broken promises

When we adopted our children thru foster care we agreed to an open relationship with mom & dad who are divorced. After 2 years of "I'll be there" and not showing up or "I'll call" and not calling we have decided to close the relationship FOR NOW. I cannot explain to these children WHY mom or dad don't show up or don't call because I don't know. I finally decided that I had to protect these children - after all we adopted them so they could live a "normal?" life. I would be very honest with her & tell her that if this continues you will not allow contact any more. good luck!!