As you will be visting sites outside London this pass may be of some use. If you search the boards you will see reviews of its value for money. It seems to be good value and is only available to visitors from outside the UK.
This seems a better link
http://www.britishheritagepass.com/ although it does look the same!
Many places offer a family ticket, but children must be under 18, sometimes under 16 in 99% of cases.
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