Whilst you're there, complete a free kids' trail or quiz and learn all about the history of the property you're visiting, play hide and seek in the grounds, and take a picnic lunch to eat al fresco.
If you want to treat yourselves and eat out, almost all National Trust cafes offer a good range of home cooked (and often locally sourced) food, including vegetarian options.
All you have to do is download a voucher, take it to a participating National Trust property and two children can receive free entry per paying adult - not bad eh?
By the way, if your kids like watching Horrible Histories, you can try to tie in an episode they've watched, with the NT property you're going to visit. Here's just a tiny selection:
Tudors - Tudor Merchant's House Pembrokeshire
Elizabethan - Hardwick Hall Derbyshire
Hanover, Georgians - Saltram Devon
Hanover, Victorians - Carlyle's House London
Stewarts, Charles II - Moseley Old Hall West Midlands Pin It