Pre-Puberty Changes

A number of other changes occur during middle childhood:

Children become stronger as their muscle mass increases.
Motor skills—in both strength and coordination—improve.
A school-age child's hair may become a little darker.
The texture and appearance of a child's skin gradually changes, becoming more like that of an adult.

First Signs of Puberty

Puberty often begins earlier than parents think:

Girls

Breast budding in girls starts around age ten, with some girls starting as early as eight and others not starting until thirteen.
Girls should be checked by their pediatrician if they begin puberty before age eight.
The peak growth period (in height, weight, muscle mass, and the like) in girls occurs about one year after puberty has begun.
Menstruation usually starts about 18 months to two years after the onset of puberty. On average, the first menses occur just before girls turn thirteen.
If a girl shows no signs of puberty by age 13, consult with your pediatrician.
See Physical Development in Girls: What to Expect.

Boys

Boys enter puberty about one year later than girls. The first sign is enlargement of the testes and a thinning and reddening of the scrotum, which happens at an average age of eleven but may occur anytime between nine to fourteen years.
Boys should be checked by their pediatrician if they begin puberty before age nine.
For boys, the peak growth period occurs about two years after the beginning of puberty.
If a boy shows no signs of puberty by age 14, consult with your pediatrician.
See Physical Development in Boys: What to Expect.

How to Discuss These Changes with Your Child

Your child needs to understand the physical changes that will occur in her body during puberty. There are many opportunities during this time of life for you to talk to your child about what she's experiencing. You should emphasize that these changes are part of the natural process of growing into adulthood, stimulated by hormones (chemicals that are produced within the body).

Keep track of your child's bodily changes, while fully respecting privacy. As the age ranges above indicate, there are wide variations of "normal" in the time when puberty begins. Remind your child that while her friends will grow at different rates, they will eventually catch up with one another.

Taken from: http://www.healthychildren.org/English/ages-stages/gradeschool/puberty/Pages/Physical-Development-of-School-Age-Children.aspx