While very small children don’t really need pocket money, once they reach school age it is a great opportunity to start teaching them about money (see ‘financial education’, below).
How much you give will depend on what you are willing to buy yourself: if you want your child to be able to afford a bag of sweets and a comic, you aren’t going to get much change out of £3 a week.
Pocket money is a good way to introduce your child to the idea of earning a reward: you could link it to good behaviour (perhaps via a star reward chart here) or perhaps to helpful tasks such as tidying their room or feeding the family pet.
The advent of pocket money will also give you an insight into a child’s financial personality: does it burn a hole in their pocket or are they happy to save it up towards a bigger item they really want?
If your child has spend-thrift tendencies then perhaps you could gently encourage them to save by only giving them a portion of their pocket money each week. You could set the rest aside in a piggy bank or perhaps put it in a savings account for them. You may find this savings reward chart useful.