Harman, I can understand you being bitter with him but it IS good that the kids see him regularly, at least.
If it were me I would stick a (polite!) note in the post to him and say that you want to be sure he knows your regular lady can't do wednesdays so he will need to make his own arrangements for the school pick up on his contact day. Tell him you are out on a course or something on a Wednesday so you need to be sure he knows this because you won't be free to help out.
You shouldn't need to tell him you'll be out, but as you say he does give the distinct impression that he will just not sort it then phone you!