sure, but I don't think the whole highway is obligated to slow down for a stopped car on the right shouler, only the lane next to the shoulder (and I would argue that this lane suddenly slowing all the way down to 20mph would create a worse hazard). theoretically the child could make it all the way to to left before being struck. I'm not saying this is likely, just that there is no speed where you can guarantee that you won't hit a pedestrian that is behaving erratically. the best you can do is make it very unlikely in most cases.