If your kids are over age 8 and highly athletic, you can usually apply the same standards as adult cross country skis. In general. experts suggest that you choose a ski that is 20 to 35cm longer than your child's height. Here are some length suggestions for various heights.
Your child's weight should also be taken into consideration. A soft flex ski will benefit a lightweight child, because it will provide proper wax adhesion on longer hills. In contrast, a heavier child might do better with a stiffer ski, since it will do a better job at gliding through deep snow.
If you want to save money by buying kids skis at a ski swap, be sure to examine them closely. Make sure that there are no splits or other types of defects in the tips or tails. The base of the ski should be flat and free of any type of blistering or prominent gouges. Look for kids skis whose edges have remained somewhat square. These skis will have better cornering performance.