Measure the toddler's lower leg. Using a measuring tape, find the distance between the toddler's lower kneecap and the floor. Record that distance. Your child should have her knee bent at roughly a 90-degree angle when sitting, so aim to have a chair that will allow her feet to just touch the floor.
Consult a chart. According to furniture supplier Worthington Direct, there are general guidelines as to chair heights. For preschoolers and kindergarteners, generally 3 to 6 years old, the average chair height is 12 inches. Children in grades one and two will most likely do well with a chair that is 14 inches high, and children in grades three to four will move up to a chair that is 16 inches high.
Measure the height of the chair. After picking a chair using a chart as a reference, take a similar measurement of the chair you have selected. You will find the distance from the highest part of the seat of the chair and then to the floor. You want to aim to keep this distance as close to the length you found of your child's lower leg.
Test the chair with your specific child in mind, because children grow at varying rates. Have him sit in the chair, with his back to the behind frame, making sure his legs can comfortably bend at 90 degrees. Have his feet touch the ground and make sure he can easily sit behind a desk or table.