To access variable from MDI child form

What data type is ActiveMdiChild? System.Windows.Forms.Form. Unless xxx is a property of the System.Windows.Forms.Form class, you must cast ActiveMdiChild to the specific type that defines the xxx property. If you need an example, please tell us which language you're using.

using Microsoft.Samples.Windows.Forms.DataGridViewSample;

frm_Materials activeForm = ActiveMdiChild as frm_Materials;
if (activeForm != null)
{
// use activeForm.northwindDS as needed
}

You don't need to know the name of the form, just its type. All your child forms should be of the same type (or inherit from a common base type). The name of that type (class) is all you need to know.