Kids love trains - especially my son and one of my best buys last year was a Family and Freinds Railcard. The card costs £30 and is valid for a year.
You can have two named adults on the card and as long as you travel with a child aged 5-15 years - adults get 1/3 off train fares and kids get 60% off.
Up to four adults and four children can travel using the card - the discount can be used on any rail fare including travel cards - so you can have a day trip to London or head to the coast with a huge discount.
You can buy these at train stations and online where I believe they are sometimes discounted at www.familyandfriends-railcard.co.uk
So, pack some lunch and head out - there's so much to explore.