Children can start learning the basics around 4 years of age. But ideally 5 to 6 is the age when they can be taught a bit more and get started with professional training.
As a parent you can:
Chess is a sport, so winning and losing is completely normal and part of the game.
Very important! A good trainer can motivate the child in the right way, teach the essentials of the game, set the right attitude, kindle the interest in learning and working individually at home. The trainer should incline your child to move towards learning so that victories come automatically but not the other way round.