Can your 6 year old who weighs 50 lbs kick with the same power as fit 22 year old at the same level who weighs 150 or 175 lbs? Could that person hold his or her own against someone that large?
She is neither expected to nor tested to.
She's not ranked in an 'art' that contains adults. You're not allowed to partake of the lessons over 7 years old. So, she's not at the same level as that 22 year old, because he couldn't hold the same rank.
It's a different system.
We're not talking piano lessons here, where there is a set expectation for everyone.
Could I, with my (above) tkd blue belt grapple up with a BJJ blue (or lower) belt? Not likely.
Could a BJJ black belt outkick me? Not likely.
So, why should my 6 year old holding a black belt in it's own style ever be compared to any other practitioner of any other system at all?
Why should her holding a black belt reflect poorly on tkd, or BJJ, or knitting?