Question regarding parent traveling alone with child.

I am in the midst of a divorce with my Mongolian wife in the U.S.A. I am concerned she may attempt to take one or both of our children (we have joint custody) to Mongolia. I have found out she has planned a trip to Mongolia July 9 to August 9, 2012. This is during (and beyond) the visit time she has with them. I would like to know if China, being a member of the Hague Convention, will ask for written notification of the other parent (me) in order to allow her to pass through China. I understand some countries ask for this and if the single parent cannot produce it they are not permitted to enter the country. Does this apply in China?