Frank, There is nothing wrong with encouraging them to spend time with their mother. There is nothing wrong with making sure they understand that you are okay with them spending time with her. Just don't try to manage it. They are old enough to decide. My son is 18 and I still encourage him to spend time with his father. He is still concerned that I am okay with it. I am not managing his relationship with his father in any way, shape or form. Reassurance goes a long way when they feel so torn.
~ swl
Look not mournfully into the past. It comes not back again. Wisely improve the present. It is thine. Go forth to meet the shadowy future, without fear. {Henry Wadsworth Longfellow}
