Where on Earth did you ever get the notion that the scripture test was NOT the very most important standard by which an apparition was judge. Any heresy at all immediately precludes any apparition from being judged as authentic.
I've just wondered about it for a long time and didn't see anything applying to the seers or visionaries in the norms or rules.
In the case of children seers, I wouldn't expect them to necessarily know or understand that scripture.
I know the church carefully evaluates what happens to whether it could be heretical or not using tradition, church doctrine and maybe scripture in general. If it is deemed heretical, it wouldn't be declared "worthy of belief".
Actually usually it's the sole responsibility of the local bishop.