It's not just for children! Particularly if you run magless (aka gravity cars) you will likely want to reduce the power.
If you are using ARC ONE you use the little dial on the side (you need to use a flathead screwdriver) and you can turn it to limit how far teh trigger can be pulled. There is a new style of analog controller coming out with a slider that can be set to 25%, 50%, 75% and 100% to limit the trigger pull.
For ARC AIR and ARC PRO you go into the driver set up and set the maximum power in 1% increments.
You can also adjust the how the power is delviered by selecting one of 4 profiles or making your own. For example I always select a profil where maximum power is not delivered right away, that way the cars don't get full power until the trigger is pressed for a few seconds.