A Lesson in the School of Life
Answer: In its campaign for pedestrian safety the authorities in Israel have come up with this slogan that is posted near schools:
“In the School of Life a child under the age of nine does not cross alone."
This warning is intended even for the child who wishes to cross at a crosswalk because he cannot be trusted to do so without harm. In the case you mention the danger is multiplied, for even adults should avoid jaywalking.
It is definitely "your business" to intervene, just as it would be in any situation where you can possibly save a life. You should approach that youngster and tell him that his parents would be very upset if he were to carry out his reckless plan. Then take him by the hand, lead him to the nearest crosswalk, and safely escort him across the street.
Not only will you prevent any possible tragedy but also you will be giving a child a valuable education for life in the school of life.