What's your child like with late nights? I would let Liebs go, especially as it would be "really" 8pm not 9, and on the arrangement that he goes straight to bed when he gets home, no messing around. But Liebs is often awake until 9pm on a school night (albeit usually in bed) with no real ill effects, he just doesn't need as much sleep as others, and therefore doesn't ever sleep in either.