You don't have to have the child form updating fields on the parent form.
Instead, on the child form, create a property that will return the value from the DateTimePicker
.
public DateTime PurchaseOrderDate
{
get { return dateTimePicker1.Value; } // your PO date picker control
}
Then on the main form, retrieve the value when the child form is closed:
using (mPOrder newPO = new mPOrder())
{
newPO.ShowDialog();
var poDate = newPO.PurchaseOrderDate;
// do something with the date - update grid or whatever
}