Low income WC here and our NT membership is one of the best value things we buy every year. Free parking in lots of places including Lake District. Lots of beautiful places to stop off for a walk and a cuppa when on long journeys (avoids depressing motorway services). Fantastic properties with fascinating stories, including TE Lawrence's cottage, Beatrix Potter's house - it's not all grand houses.
Some of the most beautiful gardens in the UK - Mount Stewart, Poolewe, Sissinghurst, Hidcote. There are pubs, lighthouses, even St Michael's Mount.
Picnic places, clean loos, tea rooms, second hand books, play areas, events.
Your kids won't be interested in history if you don't show an interest and take them to places and talk to them about the diverse history of these islands. So many stories. The NT are big on getting kids involved - Easter egg trails, school holiday and Christmas events. There's usually quiz sheets for inside the big houses and 'spot the Lego figure'.
It's up to you, obviously, but using being WC as a reason not to go to places, whether that's historic properties, museums, galleries, gardens, interesting places just seems such a cop out.