That is a pretty nice saddle and I'm sad that it isn't working out.
We used "pony saddles" - but they were older, well/hard used western style ones. One was a 10" and one was a 12". They were sized by the trees. The bars on the older saddles were made differently than they are now. Both fit the ponies nicely - even the tiny and somewhat narrow mini sized shetland. We had different western style girths made to fit several ponies - they varied by as much as 10" in length even though the ponies were only 2" different in height at that time (but western saddles can be adjusted on either side with the the girth straps, too). The first mare was 34.5" tall when our daughter started riding her. She matured at 37" at the withers, not sure what she measures as a mini. She's much rounder these days as well.
Most of the synthetic saddles today don't fit any of our shetlands or B sized minis - not in legth but in the width of the tree over the withers and back. Also, they all seem to be balanced wrong - when the girth is tightened - the rear of the saddle pops up in the back since the tree doesn't fit right on their backs. narrow or wide pony didn't seem to matter.
Same saddle, same pony, same child - different, homemade saddle pad that fit better - both at the withers of this little mare and also at her hips.
And here is the 12" seat saddle on the above pony's dam with our other daughter riding 2 yrs previous... Patty, the pony, is actually 4 yrs old here... 'Dira won't be 4 until Oct. In 1996 - I didn't have the bridles and bits sized to fit Patty. I purchased them from a Mini Tack store via mail order and made my first tiny headstalls as well.
We also had two 13" youth western saddles. They were a larger base tree to begin with and then you add the larger square skirts and they only fit the much larger and broader shetland stallion (45 1/4" tall) as well as larger shetland x's and full size horses.
two years ago, I was able to purchase that saddle back (the one at the very top) from the woman I'd sold it to years ago. It is now VERY old, and the leather is both oil logged and dry rotted. I have been taking leather working lessons and hope to take it apart in the next year and rework it myself for our granddaughters. If I do, we'll start with shorter fenders, then when the girls' grow I'd do another set - because if they are like their mom and both their aunts, their tiny bums will fit in the small seats much longer than the short stirrup lengths will last.
Incidentally, the stirrup fenders were way too long on all 3 of the saddles. We shortened them, didn't go small enough and then took them to a local leather worker and he was able to shorten them properly (when I took them out of the two smaller saddles myself, I couldn't work the straps back thru the tree and leather seat). Had to put new fenders on the saddle in the 96 pics - to make stirrups shorter.