There does not appear to be any study showing a causal or correlational link between BVO consumption and hyperactivity or ADHD.
Even if there was, orange sodas are one of many that contained BVO which means that you would expect any drink with BVO in it to correlate with ADHD not just orange flavored ones.
It could also be that fanta is caffeine free and caffeine is known to help with ADHD symptoms. So it may not be what theyre drinking so much as what they aren't.