Yep, ROBLOX toolbox, although moderated, lets a lot of user-created content slip through the cracks. In fact, i would say even up to 20-30% of the content on the toolbox has malicious scripts or unintented behaviors.
This is an issue that Roblox has failed to give a good solution to and I suggest contune to persistently contact them about. To avoid this in the future, teach your kid to only use freemodels uploaded by ROBLOX or verified developers. (There is a small checkmark near the creators name if they are a verified devforum user. Usually devforum members are very trustworthy to upload quality content. Also show him to check the like/dislike ratio and delete any scripts if the model doesnt need any to work.)
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Roblox bans children using free models with hidden malicious scripts
https://news.ycombinator.com/item?id=32303709