How to Show Only One Child Form and Closing the Other Forms

Question

I have actually read a few solutions here but many of them offer only solutions for showing one out of the two child forms. Others would only open and hide the opened form in the background if a new child form is opened. What I want is to open a child form and by opening that child form, I shall be able to close the previously opened child form.

 

 Please help me. Thanks.