A spa party for a tween works at home because the spa part is small and the party part is not. A salon books girls in blocks of time and sends everyone home at the end of the hour; a living room lets seven friends set their own pace for an evening and a night. The setup that carries it is a styled teepee sleepover, $140 per guest for the first three guests and $90 for each guest after that, plus a spa station add-on at $28 that brings the bowls, washcloths, mirrors, masks, he