how to get the distinct child field values (it is a text field) in the parent table

Hello, in order to do this, you need a parent/child relationship.

If you can isolate the RID of the child you want to lookup data into the parent (MAX RID for instance via summary field), you may then create a relationship from Child to Parent, and select the reference field of the relationship as the isolated RID, MAX RID, and lookup values.  You have to select the reference while creating the relationship. 

Also I say MAX RID, because that's the easiest summary to get. You can also filter that summary to whatever works for your use case, min, max, max with a specific status, etc. 

Its a little advanced technique, and I would say it puts some strain on performance, but I use this method quite often.

Let me know if this helps.