I agree with Shari -- many programs let your kids participate at an earlier age, if you ask nicely and explain the situation.
Where we have not been given permission to join early have been situations where the motor skills of the kids are invovled (e.g., even though my preschooler would have loved taking apart and rebuilding various mechanical devises, and had an appropriate attention span, he would not have had the hand-strength to do it himself).
Also, you might find the GT kids by just having a class that appeals mostly to GT kids. You might not have to adverstise solely to the GT crowd.
