Show posts of child cat also, when filtered by parent cat

I have created a post type Places through geodirectory plugin, in which there are 3 categories, ATM, Publix and Branch. Publix is the child category of ATM. What I want is whenever a user searches for ATM, posts with Publix cat should also get list down along with ATM. I am using GD > Search widget to show search filters.
How can I achieve this functionality?

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