Building a pivot with a massive data set

Hello all

First time poster though I have used this forum for help before and am hoping for some kind help today.

I have a large set of data that I would like to organize quickly into a pivot table. Currently, the data is set in two columns. Column A contains parent names (almost 5800) and column B contains child information. The child information can contain multiple values (in some cases hundreds of values) which are separated by a comma.

My goal is to be able to select a particular child value and return all parents which have that child.

I assume that this can be accomplished with a pivot table yet I am unclear on how to arrange the data in order to build an effective pivot.

Can anyone help?

EDIT - I have added an example file for reference