Plugin doesn't work

Yes, it can work with child theme. When using keyword search, you need to enable the Combine string search with filters above, however it will restrict the search when the keyword and filters are met.
FYI, keyword only search for post title and post content. It will not search through the taxonomy and meta field.

Note that keyword are error prone, sometime user might have typo when enter the keyword and if this is the case, possible it will return any result.
Also, keyword search have single quote (Apostrophe) ‘`’ issue as well, so be aware of this.