From the Associated Press.
Okay, she didn't just walk down to the state house by herself, but she came with her grandfather. Idaho doesn't prohibit open carry at the Legislature. The grandfather was there to support a bill that would end Idaho's discrimination against non-residents of the state and allow them to carry concealed in cities and towns.
In the 1880s, Idaho prohibited concealed carry in built-up areas (cities and towns). Idaho became a constitutional carry state in 2016, but only for residents of the state. Non-Idahoans must have a CCW to carry in urban areas.
It was probably not the best optics to have a little girl tote a rifle into a legislative hearing. AR-15s and kids have nothing to do with stopping the discrimination against non-residents. Still, in the culture of the gun it's a non-event that liberals will endlessly hand wring over.