The Jackson 5 actually made their first appearance at the Los Angeles Forum not in 1970, but nearly a year earlier on August 16, when they opened for Diana Ross and the Supremes. Two photos from this show appeared on the back of their first LP, Diana Ross Presents the Jackson 5.
They hadn't yet recorded "I Want You Back" so we can only guess what might have been on their set list that night -- likely it included "It's Your Thing," "There Was a Time," and "Simple Song/Can You Remember" -- songs they were used to doing live. Whatever they sang, the reviewer from the L.A. Times was underwhelmed by their act, and it's likely Berry Gordy decided they needed a bit more practice and polish before they went out on the tour circuit. They wouldn't appear again on the concert stage until May 1970.
I've only ever come across a few photos from the 1969 Forum appearance.
Looks like Michael and Diana were trying to push each other out of the spotlight from the very beginning.