I'm looking for advice on starting my 3 1/2 year old daughter in a western saddle. She has ridden from 10-20 minutes weekly, for about a year now. She either rides in front of me with a bareback pad or sits in my saddle as I lead our gentle old mare around. My goal is to start slow and safe of course, build her confidence, and have her increase her balance and strength ability. My question is: should we move to a child's saddle or add those child stirrups to my saddle or stay bareback? Our mare is a wide load and it is hard to fit a child's saddle to a large horse. My child has a slender build, about 32 lbs.
There are many issues I should address in starting such a young child to ride. First and foremost is that is it not a recommended sport for children that young. The risk of permanent head injury (or death) from a fall is greater than it is with older children because their heads weigh so much proportionately and their balance is extremely questionable (not to mention how high off the ground the child's head is on a full sized horse). Child advocacy groups and the American Medical Equestrian Association recommend against children this young riding. For this reason, it is difficult to find an ASTM/SEI approved equestrian helmet for a child this small, although there are a couple of companies making "toddler" helmets now.
If you have an approved equestrian helmet that fits your child correctly and you take all other precautions, like only allowing the child to ride on a lead-line with an adult at her side to aid in balance, I think it is okay to let her ride for short periods and let her learn to naturally balance on the horse.
Next we have the issue of the horse's size being appropriate to her body size. For a rider to balance effectively on the horse, her leg should reach halfway down the horse's barrel. If you are putting her on a full sized, wide horse, she will not really learn the right things to do with her body, since her legs will basically be pointed straight out. I recommend keeping your eye out for a nice medium sized pony so that your daughter, when she is ready to ride her own horse, will have an appropriately sized mount and learn to use her leg correctly.
It would be better for her to ride in a small child's saddle, but most of these are made for small horses (for the reasons stated above), which probably would not fit your horse (although as long as the saddle stays on, a 32 pound rider will not put too much stress on the horse's back). A second-choice compromise would be to use the child stirrups. Make sure you get the kind that fasten securely on the pommel and don't slide from side to side. The good kind will cost you around $50-75. There is a cheaper version, but they are not secure. Make sure that any saddle you use has safety stirrups that either breakaway or have full covers that do not allow her little foot to slip through (check carefully, many so called "safety" stirrups would still allow a small foot to slip through).
Just for the record, riding double with a small child is also not recommended because if the riders fall, the adult usually lands on top of the child, causing the potential for greater injury through crushing injuries.
I have to confess that I started my son at a VERY young age, but with serious precautions. Children aren't really ready to learn to ride until 6-8 when they have some strength, coordination and the ability to concentrate and follow directions. So be careful and take it slow and easy. Your daughter has plenty of time left to master the sport.