Praise be to Allah.
Advising and making a child aware of some issues at this young age should be done by setting out simple principles to deal with the situations that may form a threat or a danger to him, without scaring him, exaggerating, or describing possible threats and dangers in detail.
For example, we can set out some clear guidelines for the child having to do with touching his body, or interacting with strangers. So we can tell him in clear language that is appropriate to a child of his age: It is not acceptable for anyone to touch these areas of your body, or: It is not acceptable for any stranger to take you to a place you do not know without the knowledge of your parents or teachers.
Then after that, you should explain to him what he should do if he is faced with any of these situations.
So you can say to him for example: If anyone touches you in this place, push him away and get away from him at once.
Or: If someone insists on touching you in this place, then raise your voice and call one of the teachers, or run to them and tell them what happened.
And we should make sure and confirm that the child has understood what has been said to him, by asking him to repeat the instructions that he heard, and ask him about some imaginary situations connected to that.
We ask Allah, may He be exalted, to protect our sons and daughters, for He, may He be exalted, is the best of protectors and He is the Most Merciful of those who show mercy.
And Allah knows best.