Sanderstead/Warlingham borders children don't tend to meet in Croydon. Most go to Bromley or Bluewater for shopping.
Sanderstead on the Selsdon/Croydon border are more likely to hang in Croydon or Purley. Especially the boys.
Selsdon Sainburys seems to be the hangout point after school.
Riddlesdown is an outstanding secondary school but not a huge catchment area due to popularity. There is also a few private schools to choose from if that's your bag.
The girl who was stabbed was killed by an ex/ jilted boy if I remember correctly? I guess that could happen anywhere but most adults and teens avoid Croydon outside the very popular shopping times. Shame, as it could be great. I still go there for convenience but am careful.
Purley high street has a lot of charity shops. Has a huge Tescos though, bakers, PO.
Kenley is a kind of nothing town as in no heart but fine to live there.
Caterham is crowded but fairly safe.
Warlingham has some lovely spots.
Whyteleafe cheaper.
Kids tend to hang in parks.
Disclaimer: I really don't mean to offend anyone so I'm sorry if I have, I have very much generalised and each town has v nice spots and plus points, I was just trying to help using my own and friend's experiences.