Accessing child form from other child form

I have an mdi based app with a number of child forms docked in controls on the mdi form. in VB6 I could access a list box on one of the child forms with code in another child form ! In C# Vs2005 this seem's very difficult to do - I believe the accessibility has been changed to add security, but it's causing me major problems with the re-write of a VB6 app i am doing. Can anyone help

thanks

SteveT