Obviously it does happen. According to kidshealth.org very few boys and some girls go through what is called precocious puberty. It happens when children under the age of seven begin to show signs of puberty. Some signs my include breasts, and getting a period in girls. Boys voices will get deeper. Both may begin to grow pubic and underarm hair as well as develop body odor.
When you become aware of these changes in your children talking to them about it is probably also a good idea. They may become embarrassed around their friends if they are the only ones going through puberty. You may want to shave unwanted underarm hair. Kids can also be a little cruel so you may want to watch for any signs that your child is being teased because of the changes in their bodies. And definitely a talk about hygiene is a must.