Kuvira isn't quite as theatrical as Ozai when it comes to grand political gestures. It was a little quick, but I guess that's how it would go in RL too.
So, I did some digging. Jessie Flower is not reprising herbrole as Toph (though she was young Su in the flashback last season). That would be either Kate Higgins or Philece Sampler (the other must be adult Toph from the season one flashback). Clearly she did her research.
It does make sense they wouldn't bring the original back; an eleven yearnold can play a fine twelve year old but a twenty year old is a bit of a stretch for an 80 year old.