No redirect will take place if there are no results for a page with a "Page Child Redirect" attribute. So do not worry about that. This Add-On will only redirect when there are results, if not, it will go to the original page.
Of course you can. At any time you can go to your page's attributes and edit it the same way as you first added a Page Child Redirect to a page.
First: don’t panic. Go to Pages & Themes, if you’re not already there, and click the Attributes subpage. Now scroll to the very bottom of this page and under “Add Attribute” you select “Page Child Redirect Selector”. Hit “Go”. In the form, enter “page_child_redirect” as the handle and as name “Page Child Redirect” (or anything that suits you). Click “Add” in the bottom right and you’re done. You’ve set up the attribute again and can use it as you could upon first install. You of course could have uninstalled the package and installed it again, but where’s the fun in that?
Your attribute will still work as it should. Only be careful when you change this attribute AFTER already being in "full cache mode". When not using the redirect type (301 or 302), this will work as it always did and redirect you to the correct page. Otherwise, in most cases, the browser will use this 301/302 redirect and use the previously set page child redirect. You can play around with cache settings for these particular pages.