Definintely 4.12 vesc has the software necessary to control al the factors needed to make this not only slow top speed but also slow acceleration, you can get them cheap now $60 or thereabouts if yo search hard enough, they come up regularly on the parts market of this forum, HK have them quickly available here
The alternative is one of those cheap eBay ESCs, but you can’t limit them other then setting them to 6s and that can be too fast, would not advise controller modification as that will not limit the acceleration much or at all and 6s could be frightening and dangerous for a child let alone a teenager. a controller mod could fall out or fail in some way.
A cheap(ish) VESC will not be stressed by this at all and you should not experience any dreaded DRV/FET failures, especially running it in 6s. Also you get the lovely smooth FOC startup if you buy a sensored motor, again they can be had cheap, have a look at sensored motors under 200kv here