It's normal for girls to start having periods anywhere between the ages of 10-16, although some girls start even sooner than this. On average, periods start roughly two years after the breasts start developing and pubic hair appears. Girls also go through a growth spurt at this time, and can grow by as much as 10-12cm per year for the next two or three years.
Boys often start puberty slightly later - around the age of 11 - but it can happen as late as 14. However boys can experience wet dreams in the run up to puberty, so it's wise to explain that this might happen in case your son worries that he has wet the bed. The first sign of puberty in boys is the enlargement of the testes, followed by growth of the penis. Pubic hair appears soon after. Boys usually have their growth spurt between the ages of 12-14, when they will grow at a rate of around 4-6cm per year. For this reason it may take them a while to catch up with girls of the same age.