Hmmm...intriguing question. Well, just for fun, I typed in "Ooh Child" on CD Universe's section where you can look things up by giving a song title (the ALL MUSIC GUIDE's running hella slow).
You're right in that it's been remade a bunch of times--in addition to several CDs by the Five Stairsteps that have the song, CD UNIVERSE indicates that there's versions of "Ooh Child" on CDs by Valerie Carter, The Edwin Hawkins Singers, Ed Hamilton, Ramsey Lewis, Milton Nascimento, Beth Orton, Jeffrey Osbourne, Nina Simone, Dusty Springfield, John Stoddard, Lenny Williams, the Wondermints, and the 103rd Street Gospel Choir. The list there didn't say anything about whether or not a version by Marvin Gaye is available, but sure, it's possible that he could have done a version of it--after all, if you know your Motown history, you know that Berry Gordy often had his artists fill out their albums w/ remakes of other people's Rock/R&B/Soul songs and sometimes even showtunes--at least until the early '70s when Marvin and Stevie asserted more independance over their own albums...
Good question, though, and it's cool to look things up like this--you never know what you might learn, musically.
PEACE!