My 3 kids are at Hornsby and we live in the Nightingale triangle too. Loads of Hornsby families live nearby and we have 2 commons within an easy walk. We also know quite a few in the Hyde Farm estate and slightly further on is Telford Park area near Streatham but that would mean driving to school, (although many of my friends do ride shares from there). The Heaver estate is very popular too and is a slightly longer walking distance than the Nightingale area. Balham is a good station to live near as there is mainline as well as Tube from there.
Good luck! Hornsby is a great school, not that I am biased