What i'm trying to achieve is basically group my parent table records and get the max/latest out of each group so that I could fetch the appropriate child records.
Since you are looking for the max/latest, you can simply order by that specific field and select Top 1.
i'll just default to a simple query getting the parent records first and then passing those keys to the child table entity collection fetch statement (unless you can tell me this behavior is actually easy/supported)
You can use PrefetchPath, please check the link for more details and code samples.