Okay, so after checking the MUTCD, the "when children are present" sign is allowed, along with either a list of times or when flashing. One state defines it as

Okay, so after checking the MUTCD, the "when children are present" sign is allowed, along with either a list of times or when flashing. One state defines it as any of the below: > (1) School children are occupying or walking within the marked crosswalk. > (2) School children are waiting at the curb or on the shoulder of the roadway and are about to cross the roadway by way of the marked crosswalk. > (3) School children are present or walking along the roadway, either on the adjacent sidewalk or, in the absence of sidewalks, on the shoulder within the posted school speed limit zone extending 300 feet, or other distance established by regulation, in either direction from the marked crosswalk. So this would be a bit difficult to implement. Probably best handled by defaulting to slow down and let the driver override if they determine no children are present.