When you create a child form, you need to create a constructor that receives data. I helped you?
If not here is an example:
string TextboxValue = "";
public Form1()
{
InitializeComponent();
}
public Form1(string _textboxValue)
{
InitializeComponent();
TextboxValue = _textboxValue;
}
And now you have in the variable 'TextboxValue' of type string the value of the Textbox of the parent form.