Passing Variables to MDIChild form?

There are a few different ways this can be done.

My favorite way to share strings between forms is using a static variable on the form you're sharing from.

ie:
public static string SharedString = "This is a shared string.";

You can then access the string from within the parent:
private void RandomMethod(string x)
{
  SharedString = x;
}

Or from another form by using the parent forms name (this example assumes the parent form is called Form1):
private void Method2()
{
  localStringVariableX = Form1.SharedString;
}