I know this topic has come up quite a bit over the past couple years so bear with me. I have finally gotten around to testing this and I can publish a feature service with a related table and host it in ArcGIS Online. Because it's hosted, I noticed when attempting to configure the pop-ups in the map viewer I only get the option to see a count of my related records and only get to see the data through the attribute table. Has anyone been able to work around this? have I set something up wrong in the desktop environment? Any advice would be great.
I don't know of any work around with the AGOL map viewer. I assume you have a one to many relationship which is why you see the counts.
Configure pop-ups—ArcGIS Online Help | ArcGIS : With a one-to-many relationship, choose to display the related data within the pop-up window or in a table. When displaying attributes within the pop-up, string fields show the count. For numeric fields, you can choose to display the sum, min, max, average, or count.
Depending on your work case you might find this helpful.