Just curious. Why that exact saddle? My kids are now 10. I started them riding at 5. I noticed that they were happy in anything that was well-balanced. They couldn't care less about knee blocks, knee rolls, suede, etc. I was quite surpised when I put them in a 16.5" old Passier -slick as snot, flat, no blocks or rolls, too big for them - because it fit the horse best. They loved it. Perfectly balanced. I had a 14" Wintec for them: deep seat,knee blocks. Fit them and the horse great. They hated it. I have them in an ancient Stübben Siegfried right now. Leather is rotted out. Looks awful. Tree is solid, and I replaced the billets. They're in heaven. It has flawless balance.
Best kid's saddle I know of is the Stübben Rex. They're old. Some are in great shape, some need some TLC, but they put your kid in a great position. Smallest size is 15". I'd hunt for one of those.
Sattlerei Beim Kloster Schönthal and AG Felsbach also made some great kids' saddles in the 15.5" to 16" seat sizes. Very similar to the Stübben Rex. Similar balance. All have blocks and knee rolls. None have been made for at leat 30-40 years. Look at eBay, TackTrader, Horseclicks, consignmemt stores, etc.