We normally use "san" by default. If you want to refer to the children:
[Boys]
1 ~ 5 years old, "chan"
6 ~ before getting employed, "kun"
[Girls]
1 ~ 12 years old, "chan"
13 ~ , "san"
[Adult]
all "san"
The border between ages is approximate.
However, there are some exceptions. If they are younger family or younger relatives, or you feel them as if your family, you can use "kun" to male, and "chan" to female.