I don't think the OS matters here, you should teach her common sense and OPSEC, and make sure she's not exposed to weird stuff (so at that point the OS is not that relevant).
She's not going to be doing complex stuff I assume, just get her something easy (like Windows) and let her open an Excel sheet like all the other kids.
I'm going to get crucified for this, but yeah, I'm sure when she has trouble and asks her teachers on how to do something, they'll know how to guide her on TempleOS with precise instructions, while Timmy is there by her side on MSPaint drawing doodles.