Note that "joven" is not "niño". A "niño" is a human being under 12 years old, but a "joven" is a teenager or a young adult.
After "asegurar" one doesn't use "de", unless it is "asegurar
se", and the meaning changes too:
- José me aseguró que vendría. Voy a llamarlo; así me aseguro de que sí vendrá.
José assured me that he'd come. I'll call him, that way I'll make sure he will come.
I will also have to object the use of "asegurarse", because the sentence says that the program wants to motivate a child to exercise, but there is no way to control that it will, and "asegurarse" means that the responsible people will
make sure things will be as they expect.
Some proposals:
- hacer que el niño se mueva / se ejercite / haga ejercicio / realice actividades físicas...
- motivar al niño para que...
- provocar que el niño...
...