The classes for toddlers are usually 30 minutes, and as they get older to 45 minutes, then 55 minutes or an hour, and then for rec. kids it can go to 1.5 hours. The instructors are usually adults for those young kids classes because the under 18 instructors are in school - the classes are usually before 3pm for little kids. You should watch a class to check out what they do, how the instructor is. Gyms do have kids (under 18) teaching kids - they are usually gymnasts themselves, but sometimes they are not and get trained for the job. Depends on the gym. Sometimes the really experienced gymnasts who are trained to be coaches too are good, but they don't have the experience obviously that a coach with 10+ years of coaching and their own gymnastics experience has.
Toddlers do basic skills, like play with parachute, jump on trampoline, jump / stick landing, play on bars, jump on tumble track, then handstands, cartwheels, etc. Good for flexibility, hand-eye coordination, basics.