Matthew 7:18
18 A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit.
Symbolically it means God wants us to reform. I heard from Asmongold that if you mess someone up mentally, the tree will get bent. Which is a good way to put it.
But… if you took it literally. What would you tell a kid who ate rotten apple on the ground? 
It does strengthen your immune system, even if its not good to do when you get older.
Jesus said that we can tell whether people are good by the kind of fruit they bear. “God wants us to reform”: we can look at ourselves and see what kind of fruit we are bearing, and we can change so that we yield better fruit.
With a web search, I didn’t find anything saying it’s safe for kids to eat rotten apples.
It isnt safe. Thats why it builds your immune system. it toughens you! If you endure pain you wont let it affect you as much, as someone who hasnt touched it. Which will fall instantly and die.