Hmmm. I wouldn’t say stunted growth and muscle weakness was...

Hmmm. I wouldn’t say stunted growth and muscle weakness was asymptomatic, but perhaps I’m being pedantic.
Irrespective, I fail to see how testing will prevent “long Covid”, unless you mean to isolate every positive child.
The best preventative measure is vaccination and the best treatment for “long Covid” seems, again, to be vaccination.
Medical researchers suspect (but have not yet proven) that the virus is smouldering away in some part of the body (probably the endothelium). So they continue to suffer and yet continue to test negative, because the virus is not on the mucus membranes.
In those cases I suspect a blood test might be more informative.
All IMO