Just got back from holiday this weekend. I took time out last Wednesday to take my eldest daughter up Hellvelyn from Wythburn in less than ideal weather, the round trip took 4hrs including numerous chocolate breaks. Memory map says it's 8.5km with 800m of ascent. So what i here you say well she's only 6 and she walked the lot !! I very pround of her :o)
I had my other two 3 and 2 climbing on there hands and knees to get to the top of Pen Hill above Leyburn in the dales later in the week as well and they loved it.
I'd recommend sharing the hills with your kids, i had a thoroughly enjoyable time, only problem is i can't sneak off on my own anymore !!