There's some highly promising family fare this month. The Actor' Gang, sponsored by a grant from the National Endowment for the Arts, presents an outdoors production, Titus the Clownicus (Culver City's Media Park, adjacent to the Ivy Substation, August 11-September 2), presented for free, on Saturday and Sunday mornings. Also for the younger set is Make Mine Myths (Santa Monica Playhouse, August 3), a bilingual Japanese-English musical based on American and Japanese myths. Finally, Costa Mesa's South Coast Repertory offers the musical Time Again in Oz (August 11-19), Susan L. Zeder's adaptation of L. Frank Baum's classic Oz tales.