I have marketing cloud with a parent and two child business units. Multi-Org is not enabled. The parent business unit is connected to CRM and I could created journeys based on Salesforce Data Event. However when I tried to create a journey using Salesforce Data Event in child business units, I get the error "There was a problem retrieving the Integration Status".
I verified that the MC API user has access to all the business units and Not sure why it is not working. Wondering If I can create journeys in child business units using Salesforce Data Event in a Non Multi-Org environment.
I would postulate that the auth token being created is being created for API calls to your parent business unit. While the user may have permission to the child BU, authorization calls to Marketing Cloud provide tokens for a specific BU only.
Without knowing the inner workings of Marketing Cloud Connect, I can only suggest that you try creating an API user in the child BU then using those credentials for Marketing Cloud Connect. I believe this will solve your issue.
I see that you have mentioned about having Multi-org enabled.Have you actually went into the Child and Setup>Apps>Salesforce Integration to connect the Child with the Salesforce org. Multi org does not inherit from parent.