Here's a good entry from about.com on how and when to start using a deodorant.
Most kids need to begin using a deodorant when they are going through puberty. Some have noticeable body odor and need to use a deodorant every day even before they start puberty though.
Puberty
Since body odor is linked to puberty, the first step is determining if they have started puberty. Keep in mind that girls typical start puberty between the ages of 8 and 13, while boys start when they are between 9 and 14 years old. So your son would definitely be too young to be normally starting puberty already.
Signs of starting puberty might include breast development (in your daughter), pubic hair, or underarm (axillary) hair. If you do notice any of these signs, talk to your Pediatrician.
Controlling Body Odor
Whether or not they are starting puberty, if they have a 'very strong body odor,' then you want to help them control it. In addition to considering using a deodorant, you might make sure that they practice these general personal hygiene basics, including that they:
- take a daily bath or shower
- take their bath or shower in the morning, so that there are fewer bacteria on their skin to make their sweat smell
- take another bath or shower after activities and sports when they sweat a lot
- actually wash with soap and a washcloth all over, including their armpits, genitals, and feet, when they take a bath or shower
- wear clean underwear, socks, and other clothes each day
- wear loose fitting cotton clothing that may help them to sweat less
- watch their diet to see if something they are eating, like garlic, onions, or spicy food, is causing, or at least contributing, to the BO
You might also change the brand of soap they are using and even consider using an antibacterial deodorant soap, like Dial.
If these tips don't stop the BO, you may have to consider using a deodorant already. Since there aren't any deodorants that are designed for children at this age, you might consider using one for pre-teens or teens.
And since they are a little younger than average to need a deodorant, be sure to talk to your Pediatrician about this too.