We love the Lakes and I am thrilled that even now at 10 DS1 would rather go there than anywhere else on holiday. It is so lovely to see both boys (10 & 8) thoroughly enjoying simple pleasures - getting to the top, trowing stones in a stream, paddling, watching waterfalls or maybe building a dam.
They're also up for a reasonable length walk as long as there are lots of points of interest along the way (or the promise of ice cream/a pub meal at the end)
We like to make the walks seem adventurous, with a fair bit of climbing and the feel that you've "climbed a mountain" but haven't yet done any of the real Lakeland classics with the DC. They have explored Loughrigg extensively and climbed the Lion & Lamb at Grassmere and Catbells. What would be a natural progression? We tend to cover 6-8 miles in a day, depending on the gradient.