Communicating between 2 Windows Application Forms?

I have a windows app in which it has a main MDI form with many MDIChild forms. There are several instances where more than 1 form needs to be open. When I make updates on one form, they need to be reflected in the other open form. I'm assuming that won't happen until I make the other form the active form. If this is true, which Form Event should I use to make the updates, if not, what is the preferred method?

Thanks,