Kaia, a first grader at charter school, had a tantrum earlier in the day where she had kicked and punched three school employees, leading to her arrest on a charge of misdemeanor battery, according to her arrest report.

i am going to try and look at it from a policeman's POV.

if you are dealing with a kid you don't know and they are known to throw violent tantrum, why take the chance that she starts another one and might hurt herself while in your custody?

Cuffed, she is more likely to remain manageable so less chance of another violent incident and it's not going to actually harm her.

This is one of those situations where at first glance the first question that comes up is 'do police really need a pair of handcuffs to control a six year old?', but how about it 'is it really unreasonable to enact a harmless security measure to mitigate risk of needing to get into a physical tassle with a kid prone to throwing tantrums'.

Another risk is she might in fact try to bolt - kids are far more pone to irrational behavior - and now you are a chasing down a 6 year old, trying to grab her without hurting her. Depending on where she runs and how hard she tries that's a lot of potential risks that are again mitigated somewhat by the restraints.