I love Gelfand's books for kids, but by no means you can use them alone, only as a supplement to a good textbook. Gelfand wrote his books for former USSR schoolchildren, who already had a strong foundation in basic mathematical skills, and just wish to look at familiar problems from the different angle or to solve more tricky problems than those they had at school. His style is very clear and simple, and your child can just read these books alone during his free time.