In reply to Adrian Bates:
You haven't said where you'll be staying (unless I missed that) which makes giving concrete advice difficult, and I would try to avoid long drives as they won't improve the mood either, but if my kids are anything to go by, the more challenging the better. If we walk through meadows of sheep they are bored "Why do we have to do this?" but get them on Striding Edge, or Jakes Rake and they love it.
Notable things we have done over the last few years include:
a) Helm Crag above Grasemere is a lovely short walk, and there is a bit of (optional) climbing to get onto the Howitzer
https://en.wikipedia.org/wiki/Helm_Crag This has the advantage of being near to the shops for afterwards!
b) My kids liked Striding edge, but its quite a long round trip from Glenridding.
c) They also *loved* Jakes Rake, but by the time we did this they had done lots of scrambling, and several Via Ferrata. I'm not sure I'd have taken them earlier (they were 12 and 15 at the time).
d) +1 for Whinlatter forest I've ridden this with my son whilst some of the other kids (my daughter and her cousins) went to the TreeTops course.
e) Grizedale North Face MTB trail was good with my son (and on another occasion with my wife) but wild horses wouldn't make my daughter ride it.
LR