Really? What about child-child-child-child-relationships? I think it would not work to let the client handle this. It would lead to very error-prone code and unnecessary traffic. You easily forget to update all needed timestamps and are going to break your app almost always at some point in time.
Timestamps should not be handled by the client. SQL clients also doesn’t need to take care of this. The database does it. Or you can tell the database to do it by using database-triggers. I guess what comes closest to these triggers are @lambda directives. Only problem here is that the dgraph lambda server is implemented in javascript and good luck trying to run that cross-plattform. (see Why Javascript for dgraph-lambda?)