Not sure why you put “filament” in quotes in your original message. That is exactly what stays inserted in the body and NOT at all “a big piece of metal”. The needle (barely perceptible during the seconds of insertion) gets retracted into the insertion pod. Granted, I am an adult, but I do find the Freestyle Libre flash monitor to be super comfortable and painless to the point where you forget it’s on your arm. (See more about my first experience with this sensor here: FreeStyle Libre Flash Glucose Monitoring System - #25 by Dessito)
For a perspective closer to that of an actual child wearing it, see here: Free to Dream (Freestyle Libre review).
I say give your child a chance to try it. It might indeed not be the right choice for him/her, for reasons of size or other considerations, but you can’t know in advance. And more likely than not, your child will love it.