After purchasing the theme Infinity-Elite (infinityelite) you normally find the child theme as zip into the download folder. (infinityelite-child.zip)
However if you somehow lost it, here it is:
Download sample child theme: infinityelite-child.zip
To have a bigger overview about child themes please check the Codex article here: https://codex.wordpress.org/Child_Themes
If you have other questions, please feel free to contact us via email or forum!