You would need to create an instance of the childform.
So...
ChildForm cf = new ChildForm();
cf.Button1.Enabled = false;
You have to keep in mind though, it could be a different instance than the child form that is currently being shown.
To be sure, depending on your code (which I can't see) and how your program is laid out I would probably do something like this...
ChildForm cf = new ChildForm();
cf.show();
cf.Button1.Enabled = false;
so here I know that the ChildForm that is showing is the one that has the button disabled.