Once you are certain your child has chicken pox, you should quarantine them until all the lesions are crusted over. At that point, they will not be able to spread the virus.
Surprisingly, the chicken pox virus is spread through the air where it enters the lungs, then spreads through the blood and to then to the skin.
If your child comes down with the chicken pox, keep him home from creche or school until you are certain that he is no longer contagious. This usually takes between 10 to 20 days.