Our Tot Yoga (crawling-24 months) and Preschooler or Little Family Yoga (21 months-4 yrs) have a 3 month age overlap on purpose. Personality, more than physical development, should be the deciding factor between the two classes.
Tot Yoga tends to be a quieter class with more repetition from week to week, so suits more reserved toddlers better.
Preschooler / Little Family Yoga tends to be louder and move move more quickly from activity to activity to suit the energy level of 2-4 yr olds. Little Family Yoga requires more verbal direction following, incorporates more imaginative play, and typically involves less physical helping from parents. Very active almost-2 year olds often do well in this class because the social environment challenges them more than that of the younger group. Younger siblings accustomed to the energy and speed of older toddlers and preschoolers tend to enjoy this class as well, even if they are not able to do every pose.
Toddler Family Yoga (walking-4 yrs) is a crossover class which is very much like Little Family Yoga with some adaptations 12-21 month old students.