We want to create a suppression list for marketing communication. As part of which We want to find out if any Subscriber Key has received any email from any of the Child BUs in our org. Is it possible to get the data of all the sends from all the child BUs using All subscriber list from the parent BU? In the All subscriber list we were able to find out the Click and open Activity detail from the Marketing Cloud UI, but we weren't able to find the Send data from there.
Tracking data is stored in Data Views, each BU has it’s own set of Data Views.
To achieve your goal, you wold have to create a Shared Data Extension and create an automation in each BU separately, to query Data Views and update that Shared Data Extension. Then, create a separate automation in Parent BU, with a query logic to pull relevang subscribers from the Shared Data Extension and update the Supression list.