I recently bought a balance so that my daughter can learn more about physical science. The Ohaus Primer Balance is remarkably well-thought-out for children: it has large buckets to hold the weights being compared, an internal parallelogram arrangement makes it unnecessary to arange items carefully in the buckets, and the plastic is fairly rugged. With all this, it still achieves accuracy to within a gram.
A balance is better than an Archimedes scale for initial learning about weight because it lets children compare weights directly, not mediated by what to them is still an abstraction of numbers and weight-units. From this, they can eventually build up to the idea of a standard unit of weight.
So far, we've performed such experiments as: