Hi Angelina... You roused my curiosity, so I did a little poking around online this morning. There are a ton of photos of Robert Kennedy around, but none I saw matched your description. There were a few of him with kids, but those kids were all older than 3 years.
I also checked out the California State Library, the SF Library Historical Photo Collection, and the SF Gate Archives websites - zilch.
You may have to write or email the Chronicle and the Examiner in the hopes they have something in their old files.
I am quite sure there are other Outsidelanders who are far more proficient researchers than I am...maybe they can offer some further suggestions.
I wish you luck - finding that photo, if it still exists, would be a great family heirloom to own!