I used a saddle like this that can be found on ebay
: Shires Bambino First Leadline Saddle Black 10 | eBay
There are others if you just type in "Leadline saddle" and they are in 10" or 12" sizes. I used 10" seat on my daughter until she was 4/5 years old and she's always been tall for her age. Depending on the toddler's relative size, you could use one until 5/6 years old. We even showed leadline in a couple of shows as hunt seat.