My kids are 8, almost 9. I tried for a couple of years to find a Stubben Rex for them. They aren't THAT hard to find, but because they're one of the few quality kid-sized saddles, and they're no longer made, they come with a steep price. You can almost always find them on eBay
, but count on spending at least $250 for one - up to about $400. Supply and demand...
I had my kids first in a Wintec 15". They hated it. So did I. They felt clamped in. Then I had them in a 16" Collegiate pony-club A/P saddle with a deep seat and knee block and knee-rolls. It was a comfy, well-made saddle for both horse and rider, and they learned a lot in it. However, a couple months ago, I put my daughter in my 16.5" Passier from 1982 (flat, hard, and slick) and she was sooo happy. Even with it being too big for her, she rode in it better than in either of the other two.
My other daughter (same age - they're twins) rides in a Stubben Rex at lessons, and it does put her in an excellent position.
Sattlerei Kloster Beim Schonthal made a saddle that pops up in the ads now and then. It's called the Pony I, I think. Just as good as the Rex.