Since pounds and ounces are the customary measures of weight in the U.S., it's important for kids to understand how they work. Children of any age can learn to use a scale. Converting ounces and pounds requires multiplication and division, so wait for your kids to get that far in their math studies. Have your kids weigh themselves and multiply their weight by 16 to arrive at a total number of ounces. Conversely, set varying items on a kitchen scale set for ounces. Have the kids divide by 16 to determine the number of pounds and the leftover ounces.
Once your kids have the hang of converting ounces to pounds and back again, it's time to take a look at the metric system. Explain how the metric system changes its measurements in multiples of 10, 100 and 1,000. Bring out a metric scale and have the kids weigh themselves again. Have them divide their weight in grams by 1,000 to arrive at the correct number of kilograms, or multiply their weight in kilograms by 1,000 to arrive at the number of grams.
It's easiest to work with weights in whatever measure they're given, whether the customary American units or the corresponding metric units. However, sometimes it's necessary to convert from one to the other. Print off a conversion table from the Internet for the kids to refer to. Give them problems of conversion, changing grams and kilograms to pounds and ounces or vice versa. Challenge your kids to memorize the conversions, so they can do the math even when there isn't a conversion chart for reference. Offer a reward.
Demonstration is only part of the educational picture. For kids to really internalize the material, it's best to get them talking about it. Ask them which they found easier, converting between pounds and ounces or kilograms and grams. Have them discuss which level of accuracy is necessary when performing metric conversions. How many decimal points should be used in converting a recipe, for example, as opposed to a scientific experiment? When would it be appropriate to round off conversions, counting a kilogram as 2 pounds or a pound as 500 grams?