The stock trickle chargers are a cheap way to make the cars RTR... but... they're slow
. Alternate chargers can fill that battery in 30-60 minutes or faster... and different batteries can triple your run time.
A
budget multi-chemistry charger would be a good first upgrade, and then maybe get a second (higher capacity) battery down the road. Then your son could drive basically non-stop. But before investing any more money I'd live with the slow charge times for a few runs and see if he enjoys RC: no use spending more money if he may get more fun out of bow/pellet-gun.
Have fun!