I want to point out that sometimes a person is so freaked out that they become a danger to themselves, and being restrained is for their safety. But, you've already gotten you mind made up about this, even though, as has been pointed out, we don't have all the story to go along with what happened.
If the child had been restrained for his own safety, that's the story the police would have told to the press. They wouldn't have gone with "they handcuffed the kid for safety reasons
because he did match that description." (
source) We've been told why they handcuffed him,
because he matched the vague description of a suspect. This was said in a press briefing by the Chicago Police Superintendent. "They followed all the rules and protocols that we have in place. So, I’m not concerned about that at all. " Well, of course they did. Those rules and regulations are written with adults in mind, and no one who wrote them or reviewed them would have thought grown-ass police officers would be so terrified of a child half their size that they'd choose to put him in handcuffs when the rules don't dictate that they have to, but merely give them the option.
If this child had been restrained to prevent him from harming himself, that quote would have read "they handcuffed the kid for safety reasons, to prevent him from harming himself or others." They know damn well this kid was not acting in a manner that would necessitate restraint, and we can know that because they're not even trying to claim it was for the child's protection.