In my opinion, a PA can be a great resource. I haven't seen an ortho one, but my daughter regularly sees a PA in electrophysiology (cardiac specialty). The PA is directly responsible to an MD, and in our case, was completely knowledgeable and capable. Like all medical professionals, there are the great ones and the horrid ones. You may find that this PA is caring and skilled and personable.
I'd continue with the appointment, but I'd keep the idea of a second opinion in the back of my head, if I were you. I'd be aware of the follow-up care they're suggesting, as growth-plate fractures in children require some monitoring for a period of time after the cast is removed or the fracture is healed.
Ask how the PA communicates with his or her MD supervisor, and what would happen if surgery were to be required in the future.
And check out the PA or the supervising MD on healthgrades.com.
I hope the appointment goes well and that your daughter has a full recovery.