The short answer is: it depends. The best age for your children to start chess could be 5, 10 or 15.

A good place to start, although I have caveats which I’ll come to later, is with Piaget’sTheory of Cognitive Development. If you’re a teacher you’ll no doubt aware of this, but, if you’re a parent you may not be. A good place to read about Piaget’s theory might be here.

I’d consider learning the names of the pieces and how to set up the board to be suitable for children at the higher end of the Pre-Operational Stage (ages 5-7).

I’d consider playing a complete legal game, or playing minichess games, to be suitable for children in the Concrete Operational Stage (ages 7-11).

I’d consider playing a proficient game (say 1000+ rating) to be suitable for children in the Formal Operational Stage (ages 11+) – playing at this level requires complex thought processes which are, in general, beyond the typical primary school pupil.

Now for my two caveats.

1. A relatively small number of children will have a natural gift for chess which may enable them to play at a high level at a much earlier age than you would expect.
   
2. Many bright children can excel at chess at an early age if they’re playing and studying chess extensively at home – consider, for example, the Polgar sisters. If you’re ambitious for your children to excel at this in that way, and you’re aware of the possible dangers, that’s great.

My message to both parents and teachers of young children is this. If you want to introduce them to chess that’s great – it’s an amazing game. But take it seriously and do it properly or don’t do it at all. There’s no harm in children starting when they’re older, but there’s little value beyond short-term enjoyment in signing your children up for a chess club which runs once a week if you’re not providing reinforcement at home.