In a book a child received sixpence weekly allowance How much is this in US money?

Sixpence is part of the redundant British currency which ceased to be used in 1971.

In 1971, Sixpence translated to 2.5 New Pence in the new British decimal currency.

2.5 New Pence (or Sixpence) in 1971, would convert in today's values and at current exchange rates, to about 3.8 US cents.