The question of the form collaboration is pretty much empty, because right approach should start from understanding that forms classes and their instances are still classes and instances, so working with them makes no principle differences between forms and another other classes and objects. Nevertheless, the question turned out to be quite popular, so I wrote an article special for such cases:
Many Questions Answered at Once — Collaboration between Windows Forms or WPF Windows[
^].
I have no idea why did you decide to use a timer and how would you use it; but all things are done in a similar ways. I'll only note that it does not seem to make much sense. If you want to discuss it, you can comment on it or ask another question.
Finally, as to MDI, here is the idea: who needs MDI, ever? Why torturing yourself and scaring off your users?
Do yourself a great favor: do not use MDI at all. You can do much easier to implement design without it, with much better quality. MDI is highly discouraged even by Microsoft, in fact, Microsoft dropped it out of WPF and will hardly support it. More importantly, you will scare off all your users if you use MDI. Just don't. Please see:
http://en.wikipedia.org/wiki/Multiple_document_interface#Disadvantages,
How to Create MDI Parent Window in WPF?.
I can explain what to do instead. Please see my past answers:
How to Create MDI Parent Window in WPF?,
Question on using MDI windows in WPF,
MDIContainer giving error,
How to set child forms maximized, last childform minimized.
—SA