A child aged (5 -16) goes free during Kids Week (18 August - 1 September) to all the performances listed when accompanied by an adult paying full price. You can also purchase up to two additional children's tickets at half-price.
There is a limited ticket allocation per show and all offers are subject to availability and not applicable to tickets already purchased.
You pay for the top price ticket, and get a kids ticket free.
West end show listhttp://www.ticketmaster.co.uk/feature/kids_week/shows_activ_list.html
Also compare with http://london2for1.com/
as you can get discount on lower priced tickets if travelling via rail.