In my opinion, a true first pony should be the closest you can get to bombproof, adaptable and not easily upset by change in home, routine etc.
After all, you dont take a pony to a show and expect to spend a few days or weeks there for it to settle and perform.
I may be old fashioned in my views but that's what I consider a true first pony. Yes, they may be slightly nervous or out of sorts but never dangerous, which bucking is for small children.
Our first pony for my daughter was a dream on the ground but not consistent under saddle. One day he would take you through anything, the next he had a scary shadow following him.
I can't stress enough how quickly he destroyed my daughter confidence and how long it's taken to build that up again, still 2 years later she's nowhere near where she was when we got her first pony, despite having the perfect first pony now.
We tried everything with her first pony but she wasn't capable enough, he was more of a second pony and it was just the way he was wired.
Very much the same as humans, you cannot change who they really are. You can dumb down some aspects but they either have what it takes to be a first pony, or they don't.
The problem with small ponies is, they're rarely broken and brought on by children these days. More often than not its small capable adults who do this, which results in ponies not wanting little riders bouncing on them, being unbalanced, which most small kids are, and not being given correct aids.
This is the very reason why true children's first ponies are so hard to find and cost a small fortune.
You can't put a price on safety.