SCBWI does have an online forum, and it's somewhat active. I don't go there often, however, because I find AW and the Blue Boards to be far more useful and active.
SCBWI is a great organization, and I'd recommend children's book writers join. The conferences, workshops, and schmoozes are fantastic, and I love getting the SCBWI Bulletin in the mail (their print magazine, comes with subscription).
But when it comes to forums, I'd stick with AW and the Blue Boards.
Smish