Issue with visibility of Transparent child form in Win 7 OS

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C#

WinForms

I want to have a Transparent form as child to another form.

For creating a Transparent form (say this form is: NewChild), I have set the "TransparencyKey" property and "BackColor" property to same color (say Red)

Note: My parent form BackColor do not have Red color.

Now in the Parent form on some button Click, I have code like this:

NewChild child = new NewChild();
int iErro = SetParent(child.Handle, (int)this.Handle); // Set parent to this child form
child.Location = new Point(300, 300);
child.Size = new Size(300, 130);
child.Show();

I can able to see the child transparent form inside parent form in Windows 8.0, 8.1, 10.0.

BUT could not see the child form in WINDOWS 7.0.

Please let me know if this is an issue with Win 7.0 OS or any thing I am missing