I’d always suggest running the latest version of any themes and plugins where possible. As well as functional updates there may well be security updates released so you really should keep everything up to date. With that in mind I’d advise first activating the parent theme rather than your child theme and test the search functionality to ensure it is a problem with the parent theme and not the child. Then I’d look to upgrade the parent theme and fix the errors in the child theme.
If you decide to stick with the current theme version then you can overwrite the search.php by copying it to your child theme directory. If you compare your current theme’s search.php with a working theme’s file (e.g. twentyseventeen) you may be able to identify the issue. Feel free to paste the search.php using the code tags (above) as I/some one else may be able to help you find the problem.