RC vehicles that run on regular AA or AAA batteries are fine for kids, but they will run out quickly and you’ll spend a fortune replacing them.
The better quality RC rock crawlers for kids, usually have one of the following types of batteries:
Li-Ion: Lithium-ion battery.
Ni-Cd: Nickel-cadmium battery.
LiPo: Lithium-ion polymer battery.
Without going into too much detail, LiPo batteries are, arguably, the best for RC vehicles but they can be unsafe if overcharged. If you are prepared to monitor the battery handling, changing, and charging process yourself then LiPo batteries are excellent.
If you can’t guarantee that a responsible adult will be taking charge of the battery management, then Li-ion or NiCD batteries are best for your child’s RC vehicle.