Got this info from moneysavingexpert.com- free museums and 50% off exhibitions for 3 months. You have to get a special web address from the Daily Telegraph this saturday only. Still have to pay for child tickets though I think.
This is the basic info:
Free 3mth National Art Pass (worth £20)
Pass gets free entry to 200 museums for two people
Sat only
Buy the Daily Telegraph (£2) this Sat (30 Mar), and inside you?ll find a web page address to get the three month pass. It?s for new members only and you can choose either a single or double membership (a double includes cards for two adults living at the same address).
The pass gets free entry to 200 museums, art galleries and castles around the UK, including Kensington Palace (usually £15) and Shakespeare's houses and gardens (usually £22.50). See the National Art Pass website for the full list of attractions.
You won't have to enter any debit/credit card details, so there's no danger of the pass automatically renewing after the trial period.