The image of the girl in this roadsign is actually based on the designer of the sign herself, Margaret Calvert. It was first introduced in 1965, and the image itself has, on recent signs, been "re-mastered" following degradation following various re-drawings of the original. She and co-designer Jock Kinneir needed to create new signage that was suitable for the new car-based era; that could be read easily at speed, use standard conventions and typography, and introduced shape and colour to give more information. Britain's roadsigns became a world-leading example that have stood the test of time. Read more here

And remember, when approaching a school, that "School" sign is not just some amorphous, generic child; that's Margaret!

Here's a quirk for you; in Google Streetview, as you move up Northbrook Road to the north, the sign first appears to be square on to you (as you would expect); yet when you get to it, it is turned 90° and is facing the road. This is how it was when I first researched the route, and had been thus since at least Jul 2018 (according to Google's image capture label). In fact there are at least three different dates for Google's streetmap images along this short stretch of road.