I’m not sure that the seat size is as important as the flap size. For beginners I’ve always found that it is better to err on the smaller side rather than the bigger size.
The pony saddles have shorter flaps and it helps the kids get their legs around the horse than just having 99% of their legs on the flaps.
The bigger seat size also isn’t nearly as secure as a smaller seat size.
I found when my girls were small that the Pessoa pony saddles held their value very well and were very easy to resell after they outgrew them. I even made some money on a couple of the saddles.
I know that parents might not want to purchase a saddle but the Pessoa (or Bevals or like saddles) will be easy to resell and will hold their value as long as the kids take care of them. They are pretty hardy saddles, I can’t say that my girls were great about care but they did clean up well when I conditioned them etc.
It’s really hard for the kids to learn to ride without tack that fits.
Pessoa makes a 15 3/4" seat and it would probably work really well for both girls. It will be a little big for the 6 year old but should last the older one until she’s ready to fit into an adult saddle.
Both of my girls went from a 15 3/4 to a 16.5 or 17" saddle (depending on the brand) with ease. The 15 3/4 had a little shorter flaps than an adult 16" and the flap size is what they actually outgrew first, not so much the seat because they were both really thin at that age.
I’d personally much rather be in a saddle which is too small and flaps that are too short than in a saddle which is too big. I’m much more stable that way.