My 4 year old nephew is getting more into riding, and I am thinking it's time he got his own saddle. My BO has a horse sized youth saddle, but is too narrow for Remy (Morgan). I see a lot of pony saddles, but haven't seen anything in my searches for something wider to fit the stocky horses. I don't need anything fancy, just clean and safe.
Find out if the BO saddle is semi QH bars or Full QH bars, so you know what to look for that will fit your horse. Or try on a number of Western saddles at the Tack store to see what actually fits horse well, get those measurements to hunt out other saddles.
I got a real nice kid saddle, 12" seat, by Billy Royal, that fit the wide old mare, kept the kids seated correctly over their feet. I just started asking around if anyone had heard of a kid-sized, Western horse saddle for sale, needed a wide tree for a wide horse. I got a couple names that I called to ask, and got mine for about $400, which I figured was a good price on a quality item. Good leather, built like an adult saddle of fair quality, had some weight to it, along with a bit of silver trim. Both kids used it, got a lot of miles in it, sold it for the same money about 15 years later! The boy got more time, he just didn't grow until about age 13, and I put longer fenders on it for his length of leg. With no hips on my son, the small seat size still fit well! Daughter outgrew it by age 10yrs, but she was big for her age, so we sold it on.
Maybe some "Wanted" ads at the tack store, local horse newspaper, could get you a saddle. I see a TON of those bitty seat saddles on Ebay, with good names on them too for quality. Barrel Racing is especially a good search place to find little seats with all the small girls running now. Barrel horses are usually pretty wide shouldered, so you could have the seat size needed on a tree that will fit your animal. Ask buyer questions, if the tree is not listed in the description for model and width.
I am a real stickler for having the seat size fit the kid correctly, not buying a saddle to "grow into". Kids develop so many equitiation problems, don't have fun riding, when they slop around in their saddle that is too big, both English and Western. Hope you find a nice saddle for the kid AND the horse.