My 5 year kid rides bicycle in our street which is common in our locality, sometimes he tend to go for fast pedaling, which may harm himself and others as well, my question is how to make the bicycle go slower all the times for the sake of safety for everybody?
Most bikes for small children only have a single gear which severely limits the speeds you can (comfortably) cycle at.
With a typical 28t chainring in the front, a 15t sprocket in the back and 16 inch wheels the maximum speed you can reach with fast pedaling (110rpm cadence) is about 16km/h. (minimum speed is around 11km/h at 75rpm, no wonder kids have to get off and push at steeper inclines)
16km/h is not terribly fast, that’s a speed a slightly older child can achieve on foot. Make him wear a helmet (properly! I see way too many children who wear it way too loose and at the back of the head) and the worst that’s going to happen is a scraped knee or palm. That’s assuming the area he’s riding in is safe, which would be a much bigger concern for me.
If you really wanted to you could change the gear ratio so he has to pedal even faster to reach speed. But that would ruin all the fun and utility. Right now his “comfortable” speed at the typical 90rpm pedaling cadence is already only 13km/h.