According to https://en.help.roblox.com/hc/en-us/articles/203312980 it's a fairly simple uninstall, unlike some.
In your son's login, Command -clicking on the Dock (Task bar) icon should open a Finder window showing the application, from where you can delete it. You can probably also prohibit that app from System Preferences - Parental Controls, assuming they are enabled for his account (probably, since you mention limited access & can't change the Dock).
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Failing that, and presuming some urgency … You might get a copy of Find any File , enter the search term, then hold the alt/option key when clicking the Find button.
You should be asked to provide an admin name & password.
On default settings, it should show you every file or folder with your search term in the name, in any User or other folder on the computer, and allow you to trash or delete immediately, with a ctrl-click on one or more results in the list. Take care here : you may not be asked again for an admin name/pass, and although you can 'put back' from the Trash, 'Delete Immediately' is final. Only remove what the uninstall instructions say, not every last mention of it.
That will leave the Dock icon in his account, but it won't work.