An RPE, Rapid Palate Expander is usually used for expansion for a narrow palate... widening of the upper Maxllary arch. The reason it is placed after baby are all out and permanent teeth are present, is for anchoring the appliance in place. the molars and bicusped are used for this.. RPE is cemented in place.. Usually for the first week or two the Rpe is adjusted with a key at home. Now if your child has baby teeth, he can have expansion done with a removable appliance, one you can adjust in your hand, but the child must wear all the time, or appiance won't fit because adjustments will make expander bigger.. Hope this was some help and not too confusing.. Usually a class three bite is challenging... The earlier he gets started the better.. if Orthodontist wants to wait until his baby teeth are out, get a second opinion.. The lower jaw is bigger than the upper jaw now and is going to continue to grow, so don't waste time..