Think children shouldn’t be looking at power, and especially not at W/kg. Both change a lot due to them growing and trying to push high power at a young age can cause damage to joints etc. Better to keep cycling fun and work on skills, like balance and cadence and all sorts of tricks.
Appreciate though that you need to find some sort of power level for her to have appropriatly scaled workouts indoors, just wanted to caution against looking at her power like you’d do for yourself.