What size saddles do you have for your little ponies around 11.2? My saddler suggested 14" (having not seen the pony) but I'm worried my daughter will grow out of that a bit quickly. She's just turned 5.
You can only fit a saddle to the length available, however all saddles, even models within a brand and sometimes even on exactly the same tree (cantles can be added, reshaped etc) they have different amounts of space for the rider, and take up a different amount of space on the horse's back. Unless you have a saddle with a forward balance (very rare with pony saddles, especially British brands) then the saddle can't extend beyond the back rib. You need to have the pony seen in the flesh to really know, there's a guide here https://www.ahsaddles.com/post/2019/04/30/horse-shopping-saddle-length-part-1 (there's a part 2) but of course you'd not necessarily want to rely on sellers measuring the saddle correctly.
I'm an adult (albiet short-ish) and I ride easily in a 16" saddle as my pony won't take any longer. It's probably a smidge to small for me, but it doesn't bother me. I'd assume a 5yo would be fine in 14" saddle for years- they'll likely outgrow the pony sooner than the saddle!
Really depends on the pony and the saddle, we have a reasonably short coupled 11.2 in a 14" and another 11.2 who is longer in the back in a 15" and still room to spare! Growing up I managed to ride ponies in a 15" saddle until I was a very leggy 16 year old (I am now 5'10") although I was very slim back then! So your daughter should have years left in a 14" I imagine!