'Junior' discs are smaller discs that still have the flight characteristics of the standard full size discs. These are not designed as markers or novelty toys but instead are designed for youth disc golf providing flight numbers and options just like the full size discs.
The junior sized discs are in the 50-90g weight range and measure just about 6" in diameter. This smaller diameter and weight is perfect for elementary school aged kids from 4 to 11. There's even a matching Junior Disc Golf Basket proportionally sized to the junior discs.
Junior discs are a perfect way for elementary for schools and other youth organizations to introduce disc golf to the kids at a lower cost with a product perfectly sized for their enjoyment and abilities.
For adults, these things are just fun to throw around. Or yes, you could use them as markers too.