That being said...... I will never understand how glass and used needles end up in the sandbox....
During daytime it's for small kids, nighttime it's either big kids (drinking beer/soda and smashing the bottles) or junkies doing drugs, since the playground is usually empty by then and also slightly off the main routes.
(Some sad people will even take out their anger at society by trying to hurt the innocent.)
And at all times the sandbox is also there for the cats to do their thing.